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

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

third_party/git/Documentation/user-manual.conf

@@ -0,0 +1,21 @@
+[titles]
+	underlines="__","==","--","~~","^^"
+
+[attributes]
+caret=^
+startsb=[
+endsb=]
+tilde=~
+
+[linkgit-inlinemacro]
+<ulink url="{target}.html">{target}{0?({0})}</ulink>
+
+ifdef::backend-docbook[]
+# "unbreak" docbook-xsl v1.68 for manpages. v1.69 works with or without this.
+[listingblock]
+<example><title>{title}</title>
+<literallayout class="monospaced">
+|
+</literallayout>
+{title#}</example>
+endif::backend-docbook[]