pull --rebase=<type>: allow single-letter abbreviations for the type

pull --rebase=<type>: allow single-letter abbreviations for the type

Git for Windows' original 4aa8b8c8283 (Teach 'git pull' to handle
--rebase=interactive, 2011-10-21) had support for the very convenient
abbreviation

	git pull --rebase=i

which was later lost when it was ported to the builtin `git pull`, and
it was not introduced before the patch eventually made it into Git as
f5eb87b98dd (pull: allow interactive rebase with --rebase=interactive,
2016-01-13).

However, it is *really* a useful short hand for the occasional rebasing
pull on branches that do not usually want to be rebased.

So let's reintroduce this convenience, at long last.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>