SubmittingPatches: write problem statement in the log in the present tense

SubmittingPatches: write problem statement in the log in the present tense

We give a guidance for proposed log message to write problem
statement first, followed by the reasoning behind, and recipe for,
the solution.  Clarify that we describe the situation _before_ the
proposed patch is applied in the present tense (not in the past
tense e.g. "we used to do X, but thanks to this commit we now do Y")
for consistency.

Signed-off-by: Junio C Hamano <gitster@pobox.com>