log: add log.showSignature configuration variable

log: add log.showSignature configuration variable

Users may want to always use "--show-signature" while using git-log and
related commands.

When log.showSignature is set to true, git-log and related commands will
behave as if "--show-signature" was given to them.

Note that this config variable is meant to affect git-log, git-show,
git-whatchanged and git-reflog. Other commands like git-format-patch,
git-rev-list are not to be affected by this config variable.

Signed-off-by: Mehul Jain <mehul.jain2029@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>