SubmittingPatches: Cite the 50 char subject limit

SubmittingPatches: Cite the 50 char subject limit

Change the SubmittingPatches recommendations to mention the 50
character soft limit on patch subject lines. 50 characters is the soft
limit mentioned in git-commit(1) and gittutorial(7), it's also the
point at which Gitweb, GitHub and various other Git front ends start
abbreviating the commit message.

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