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

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

third_party/git/Documentation/git-merge-tree.txt

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+git-merge-tree(1)
+=================
+
+NAME
+----
+git-merge-tree - Show three-way merge without touching index
+
+
+SYNOPSIS
+--------
+[verse]
+'git merge-tree' <base-tree> <branch1> <branch2>
+
+DESCRIPTION
+-----------
+Reads three tree-ish, and output trivial merge results and
+conflicting stages to the standard output.  This is similar to
+what three-way 'git read-tree -m' does, but instead of storing the
+results in the index, the command outputs the entries to the
+standard output.
+
+This is meant to be used by higher level scripts to compute
+merge results outside of the index, and stuff the results back into the
+index.  For this reason, the output from the command omits
+entries that match the <branch1> tree.
+
+GIT
+---
+Part of the linkgit:git[1] suite