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

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

third_party/git/check-builtins.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+
+{
+	cat <<\EOF
+sayIt:
+	$(foreach b,$(BUILT_INS),echo XXX $(b:$X=) YYY;)
+EOF
+	cat Makefile
+} |
+make -f - sayIt 2>/dev/null |
+sed -n -e 's/.*XXX \(.*\) YYY.*/\1/p' |
+sort |
+{
+    bad=0
+    while read builtin
+    do
+	base=$(expr "$builtin" : 'git-\(.*\)')
+	x=$(sed -ne 's/.*{ "'$base'", \(cmd_[^, ]*\).*/'$base'	\1/p' git.c)
+	if test -z "$x"
+	then
+		echo "$base is builtin but not listed in git.c command list"
+		bad=1
+	fi
+	for sfx in sh perl py
+	do
+		if test -f "$builtin.$sfx"
+		then
+			echo "$base is builtin but $builtin.$sfx still exists"
+			bad=1
+		fi
+	done
+    done
+    exit $bad
+}