merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/contrib/hooks/multimail/README.migrate-from-post-receive-email

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+git-multimail is close to, but not exactly, a plug-in replacement for
+the old Git project script contrib/hooks/post-receive-email.  This
+document describes the differences and explains how to configure
+git-multimail to get behavior closest to that of post-receive-email.
+
+If you are in a hurry
+=====================
+
+A script called migrate-mailhook-config is included with
+git-multimail.  If you run this script within a Git repository that is
+configured to use post-receive-email, it will convert the
+configuration settings into the approximate equivalent settings for
+git-multimail.  For more information, run
+
+    migrate-mailhook-config --help
+
+
+Configuration differences
+=========================
+
+* The names of the config options for git-multimail are in namespace
+  "multimailhook.*" instead of "hooks.*".  (Editorial comment:
+  post-receive-email should never have used such a generic top-level
+  namespace.)
+
+* In emails about new annotated tags, post-receive-email includes a
+  shortlog of all changes since the previous annotated tag.  To get
+  this behavior with git-multimail, you need to set
+  multimailhook.announceshortlog to true:
+
+      git config multimailhook.announceshortlog true
+
+* multimailhook.commitlist -- This is a new configuration variable.
+  Recipients listed here will receive a separate email for each new
+  commit.  However, if this variable is *not* set, it defaults to the
+  value of multimailhook.mailinglist.  Therefore, if you *don't* want
+  the members of multimailhook.mailinglist to receive one email per
+  commit, then set this value to the empty string:
+
+      git config multimailhook.commitlist ''
+
+* multimailhook.emailprefix -- If this value is not set, then the
+  subjects of generated emails are prefixed with the short name of the
+  repository enclosed in square brackets; e.g., "[myrepo]".
+  post-receive-email defaults to prefix "[SCM]" if this option is not
+  set.  So if you were using the old default and want to retain it
+  (for example, to avoid having to change your email filters), set
+  this variable explicitly to the old value:
+
+      git config multimailhook.emailprefix "[SCM]"
+
+* The "multimailhook.showrev" configuration option is not supported.
+  Its main use is obsoleted by the one-email-per-commit feature of
+  git-multimail.
+
+
+Other differences
+=================
+
+This section describes other differences in the behavior of
+git-multimail vs. post-receive-email.  For full details, please refer
+to the main README file:
+
+* One email per commit.  For each reference change, the script first
+  outputs one email summarizing the reference change (including
+  one-line summaries of the new commits), then it outputs a separate
+  email for each new commit that was introduced, including patches.
+  These one-email-per-commit emails go to the addresses listed in
+  multimailhook.commitlist.  post-receive-email sends only one email
+  for each *reference* that is changed, no matter how many commits
+  were added to the reference.
+
+* Better algorithm for detecting new commits.  post-receive-email
+  processes one reference change at a time, which causes it to fail to
+  describe new commits that were included in multiple branches.  For
+  example, if a single push adds the "*" commits in the diagram below,
+  then post-receive-email would never include the details of the two
+  commits that are common to "master" and "branch" in its
+  notifications.
+
+      o---o---o---*---*---*    <-- master
+                       \
+                        *---*  <-- branch
+
+  git-multimail analyzes all reference modifications to determine
+  which commits were not present before the change, therefore avoiding
+  that error.
+
+* In reference change emails, git-multimail tells which commits have
+  been added to the reference vs. are entirely new to the repository,
+  and which commits that have been omitted from the reference
+  vs. entirely discarded from the repository.
+
+* The environment in which Git is running can be configured via an
+  "Environment" abstraction.
+
+* Built-in support for Gitolite-managed repositories.
+
+* Instead of using full SHA1 object names in emails, git-multimail
+  mostly uses abbreviated SHA1s, plus one-line log message summaries
+  where appropriate.
+
+* In the schematic diagrams that explain non-fast-forward commits,
+  git-multimail shows the names of the branches involved.
+
+* The emails generated by git-multimail include the name of the Git
+  repository that was modified; this is convenient for recipients who
+  are monitoring multiple repositories.
+
+* git-multimail allows the email "From" addresses to be configured.
+
+* The recipients lists (multimailhook.mailinglist,
+  multimailhook.refchangelist, multimailhook.announcelist, and
+  multimailhook.commitlist) can be comma-separated values and/or
+  multivalued settings in the config file; e.g.,
+
+      [multimailhook]
+              mailinglist = mr.brown@example.com, mr.black@example.com
+              announcelist = Him <him@example.com>
+              announcelist = Jim <jim@example.com>
+              announcelist = pop@example.com
+
+  This might make it easier to maintain short recipients lists without
+  requiring full-fledged mailing list software.
+
+* By default, git-multimail sets email "Reply-To" headers to reply to
+  the pusher (for reference updates) and to the author (for commit
+  notifications).  By default, the pusher's email address is
+  constructed by appending "multimailhook.emaildomain" to the pusher's
+  username.
+
+* The generated emails contain a configurable footer.  By default, it
+  lists the name of the administrator who should be contacted to
+  unsubscribe from notification emails.
+
+* New option multimailhook.emailmaxlinelength to limit the length of
+  lines in the main part of the email body.  The default limit is 500
+  characters.
+
+* New option multimailhook.emailstrictutf8 to ensure that the main
+  part of the email body is valid UTF-8.  Invalid characters are
+  turned into the Unicode replacement character, U+FFFD.  By default
+  this option is turned on.
+
+* Written in Python.  Easier to add new features.