patman: Don't barf if the word 'commit' starts a line

Patman's regular expression for detecting the start of a
commit in a git log was a little simplistic and could be
confused if the git log itself had the word "commit" as
the start of a line (as this commit does).  Make patman
a little more robust.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>