log: add -P as a synonym for --perl-regexp

log: add -P as a synonym for --perl-regexp

Add a short -P option as a synonym for the longer --perl-regexp, for
consistency with the options the corresponding grep invocations
accept.

This was intentionally omitted in commit 727b6fc3ed ("log --grep:
accept --basic-regexp and --perl-regexp", 2012-10-03) for unspecified
future use.

Make it consistent with "grep" rather than to keep it open for future
use, and to avoid the confusion of -P meaning different things for
grep & log, as is the case with the -G option.

As noted in the aforementioned commit the --basic-regexp option can't
have a corresponding -G argument, as the log command already uses that
for -G<regex>.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>