subtree: performance improvement for finding unexpected parent commits

After testing a previous patch at larger scale, a performance issue was
detected when using git show to locate parent revisions, with a single
run of the git show command taking 2 seconds or longer in a complex repo.
When the command is required tens or hundreds of times in a run of the
script, the additional wait time is unaccepatable. Replacing the command
with git rev-parse resulted in significantly increased performance, with
the command in question returning instantly.

Signed-off-by: Roger Strain <rstrain@swri.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.sh

@@ -633,7 +633,7 @@ process_split_commit () {
 	else
 		# processing commit without normal parent information;
 		# fetch from repo
-		parents=$(git show -s --pretty=%P "$rev")
+		parents=$(git rev-parse "$rev^@")
 		extracount=$(($extracount + 1))
 	fi