scripts: Fix git-checkout.sh to work with non-master branches (take 2)

First, rename $TAG to $COMMITTISH.  We already pass tags, branches (well, only
master) and full SHAs into this script.

Xen uses master for QEMU_UPSTREAM_REVISION, and has done for other trees too
in the path.  Apparently we've never specified a different branch, because the
git-clone rune only pulls in the master branch; it does not pull in diverging
branches.

Fix this by performing an explicit fetch of the $COMMITTISH, then checking out
the dummy branch from the FETCH_HEAD.

Suggested-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>

scripts/git-checkout.sh

@@ -1,12 +1,12 @@
 #!/bin/sh
 
 if test $# -lt 3; then
-	echo "Usage: $0 <tree> <tag> <dir>"
+	echo "Usage: $0 <tree> <committish> <dir>"
 	exit 1
 fi
 
 TREE=$1
-TAG=$2
+COMMITTISH=$2
 DIR=$3
 
 set -e
@@ -15,10 +15,11 @@ if test \! -d $DIR-remote; then
 	rm -rf $DIR-remote $DIR-remote.tmp
 	mkdir -p $DIR-remote.tmp; rmdir $DIR-remote.tmp
 	$GIT clone $TREE $DIR-remote.tmp
-	if test "$TAG" ; then
+	if test "$COMMITTISH" ; then
 		cd $DIR-remote.tmp
+		$GIT fetch origin "$COMMITTISH"
 		$GIT branch -D dummy >/dev/null 2>&1 ||:
-		$GIT checkout -b dummy $TAG
+		$GIT checkout -b dummy FETCH_HEAD
 		cd -
 	fi
 	mv $DIR-remote.tmp $DIR-remote