git: add -P as a short option for --no-pager

git: add -P as a short option for --no-pager

It is possible to configure 'less', the pager, to use an alternate
screen to show the content, for example, by setting LESS=RS in the
environment. When it is closed in this configuration, it switches
back to the original screen, and all content is gone.

It is not uncommon to request that the output remains visible in
the terminal. For this, the option --no-pager can be used. But
it is a bit cumbersome to type, even when command completion is
available. Provide a short option, -P, to make the option more
easily accessible.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>