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

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

third_party/git/Documentation/RelNotes/1.7.2.1.txt

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+Git v1.7.2.1 Release Notes
+==========================
+
+Fixes since v1.7.2
+------------------
+
+ * "git instaweb" wasn't useful when your Apache was installed under a
+   name other than apache2 (e.g. "httpd").
+
+ * Similarly, "git web--browse" (invoked by "git help -w") learned that
+   chrome browser is sometimes called google-chrome.
+
+ * An overlong line after ".gitdir: " in a git file caused out of bounds
+   access to an array on the stack.
+
+ * "git config --path conf.var" to attempt to expand a variable conf.var
+   that uses "~/" short-hand segfaulted when $HOME environment variable
+   was not set.
+
+ * Documentation on Cygwin failed to build.
+
+ * The error message from "git pull blarg" when 'blarg' is an unknown
+   remote name has been improved.
+
+And other minor fixes and documentation updates.