merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/t/lib-httpd/apply-one-time-perl.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+# If "one-time-perl" exists in $HTTPD_ROOT_PATH, run perl on the HTTP response,
+# using the contents of "one-time-perl" as the perl command to be run. If the
+# response was modified as a result, delete "one-time-perl" so that subsequent
+# HTTP responses are no longer modified.
+#
+# This can be used to simulate the effects of the repository changing in
+# between HTTP request-response pairs.
+if test -f one-time-perl
+then
+	LC_ALL=C
+	export LC_ALL
+
+	"$GIT_EXEC_PATH/git-http-backend" >out
+	perl -pe "$(cat one-time-perl)" out >out_modified
+
+	if cmp -s out out_modified
+	then
+		cat out
+	else
+		cat out_modified
+		rm one-time-perl
+	fi
+else
+	"$GIT_EXEC_PATH/git-http-backend"
+fi