patman: Check commit_match before stripping leading whitespace

True commit lines start at column zero.  Anything that is indented
is part of the commit message instead.  I noticed this by trying to
run buildman with commit e3a4facd
as master, which contained a reference to a Linux commit inside
the commit message.  ProcessLine saw that as a genuite commit
line, and thus buildman tried to build it, and died with an
exception because that SHA is not present in the U-Boot tree.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Simon Glass <sjg@chromium.org>