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>

Documentation/SubmittingPatches

@@ -142,6 +142,13 @@ The body should provide a meaningful commit message, which:
 
 . alternate solutions considered but discarded, if any.
 
+[[present-tense]]
+The problem statement that describes the status quo is written in the
+present tense.  Write "The code does X when it is given input Y",
+instead of "The code used to do Y when given input X".  You do not
+have to say "Currently"---the status quo in the problem statement is
+about the code _without_ your change, by project convention.
+
 [[imperative-mood]]
 Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
 instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy