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

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

third_party/git/Documentation/git-symbolic-ref.txt

@@ -0,0 +1,69 @@
+git-symbolic-ref(1)
+===================
+
+NAME
+----
+git-symbolic-ref - Read, modify and delete symbolic refs
+
+SYNOPSIS
+--------
+[verse]
+'git symbolic-ref' [-m <reason>] <name> <ref>
+'git symbolic-ref' [-q] [--short] <name>
+'git symbolic-ref' --delete [-q] <name>
+
+DESCRIPTION
+-----------
+Given one argument, reads which branch head the given symbolic
+ref refers to and outputs its path, relative to the `.git/`
+directory.  Typically you would give `HEAD` as the <name>
+argument to see which branch your working tree is on.
+
+Given two arguments, creates or updates a symbolic ref <name> to
+point at the given branch <ref>.
+
+Given `--delete` and an additional argument, deletes the given
+symbolic ref.
+
+A symbolic ref is a regular file that stores a string that
+begins with `ref: refs/`.  For example, your `.git/HEAD` is
+a regular file whose contents is `ref: refs/heads/master`.
+
+OPTIONS
+-------
+
+-d::
+--delete::
+	Delete the symbolic ref <name>.
+
+-q::
+--quiet::
+	Do not issue an error message if the <name> is not a
+	symbolic ref but a detached HEAD; instead exit with
+	non-zero status silently.
+
+--short::
+	When showing the value of <name> as a symbolic ref, try to shorten the
+	value, e.g. from `refs/heads/master` to `master`.
+
+-m::
+	Update the reflog for <name> with <reason>.  This is valid only
+	when creating or updating a symbolic ref.
+
+NOTES
+-----
+In the past, `.git/HEAD` was a symbolic link pointing at
+`refs/heads/master`.  When we wanted to switch to another branch,
+we did `ln -sf refs/heads/newbranch .git/HEAD`, and when we wanted
+to find out which branch we are on, we did `readlink .git/HEAD`.
+But symbolic links are not entirely portable, so they are now
+deprecated and symbolic refs (as described above) are used by
+default.
+
+'git symbolic-ref' will exit with status 0 if the contents of the
+symbolic ref were printed correctly, with status 1 if the requested
+name is not a symbolic ref, or 128 if another error occurs.
+
+GIT
+---
+Part of the linkgit:git[1] suite