blame: use .mailmap unconditionally

blame: use .mailmap unconditionally

There really isn't any point in turning off .mailmap.  The
number of mailmap lookups are bounded by number of lines in the
target file, and the real blame processing is much more
expensive.  If it turns out to be too costly, we should optimize
the mailmap lookup itself, instead of avoiding the call.

If the author information of commits of the project are
relatively clean, .mailmap would have only small number of
entries, and the overhead of looking it up will not be high.  On
the other hand, if the author information is really screwed up
that a good .mailmap needs to be maintained to run shortlog,
giving uncleaned names in blame output is not helpful at all
either.

Signed-off-by: Junio C Hamano <junkio@cox.net>