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

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

third_party/git/Documentation/RelNotes/1.8.4.1.txt

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+Git v1.8.4.1 Release Notes
+==========================
+
+Fixes since v1.8.4
+------------------
+
+ * Some old versions of bash do not grok some constructs like
+   'printf -v varname' which the prompt and completion code started
+   to use recently.  The completion and prompt scripts have been
+   adjusted to work better with these old versions of bash.
+
+ * In FreeBSD's and NetBSD's "sh", a return in a dot script in a
+   function returns from the function, not only in the dot script,
+   breaking "git rebase" on these platforms (regression introduced
+   in 1.8.4-rc1).
+
+ * "git rebase -i" and other scripted commands were feeding a
+   random, data dependant error message to 'echo' and expecting it
+   to come out literally.
+
+ * Setting the "submodule.<name>.path" variable to the empty
+   "true" caused the configuration parser to segfault.
+
+ * Output from "git log --full-diff -- <pathspec>" looked strange
+   because comparison was done with the previous ancestor that
+   touched the specified <pathspec>, causing the patches for paths
+   outside the pathspec to show more than the single commit has
+   changed.
+
+ * The auto-tag-following code in "git fetch" tries to reuse the
+   same transport twice when the serving end does not cooperate and
+   does not give tags that point to commits that are asked for as
+   part of the primary transfer.  Unfortunately, Git-aware transport
+   helper interface is not designed to be used more than once, hence
+   this did not work over smart-http transfer.  Fixed.
+
+ * Send a large request to read(2)/write(2) as a smaller but still
+   reasonably large chunks, which would improve the latency when the
+   operation needs to be killed and incidentally works around broken
+   64-bit systems that cannot take a 2GB write or read in one go.
+
+ * A ".mailmap" file that ends with an incomplete line, when read
+   from a blob, was not handled properly.
+
+ * The recent "short-cut clone connectivity check" topic broke a
+   shallow repository when a fetch operation tries to auto-follow
+   tags.
+
+ * When send-email comes up with an error message to die with upon
+   failure to start an SSL session, it tried to read the error
+   string from a wrong place.
+
+ * A call to xread() was used without a loop to cope with short
+   read in the codepath to stream large blobs to a pack.
+
+ * On platforms with fgetc() and friends defined as macros, the
+   configuration parser did not compile.
+
+ * New versions of MediaWiki introduced a new API for returning
+   more than 500 results in response to a query, which would cause
+   the MediaWiki remote helper to go into an infinite loop.
+
+ * Subversion's serf access method (the only one available in
+   Subversion 1.8) for http and https URLs in skelta mode tells its
+   caller to open multiple files at a time, which made "git svn
+   fetch" complain that "Temp file with moniker 'svn_delta' already
+   in use" instead of fetching.
+
+
+Also contains a handful of trivial code clean-ups, documentation
+updates, updates to the test suite, etc.