lib/oe/reproducible: Use git log without gpg signature

Previously, if "showSignature" is present in user gitconfig, parsing
of the timestamp will fail.

Ideally we should replace this command with a git plumbing command.

Signed-off-by: Benoît Mauduit <bmauduit@beneth.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>