Before the grep_source interface existed, grep_buffer was
used by two types of callers:

  1. Ones which pulled a file into a buffer, and then wanted
     to supply the file's name for the output (i.e.,
     git grep).

  2. Ones which really just wanted to grep a buffer (i.e.,
     git log --grep).

Callers in set (1) should now be using grep_source. Callers
in set (2) always pass NULL for the "name" parameter of
grep_buffer. We can therefore get rid of this now-useless
parameter.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>