If log.showsignature is true (or --show-signature is passed) while
performing a `subtree add` or `subtree pull`, the command fails.
toptree_for_commit() calls `log` and passes the output to `commit-tree`.
If this output shows the GPG signature data, `commit-tree` throws a
fatal error.
This commit fixes the issue by adding --no-show-signature to `log` calls
in a few places, as well as using the more appropriate `rev-parse`
instead where possible.
Signed-off-by: Stephen R Guglielmo <srg@guglielmo.us> Signed-off-by: Junio C Hamano <gitster@pobox.com>