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>