pickaxe: merge diffcore_pickaxe_grep() and diffcore_pickaxe_count() into diffcore_pic...

pickaxe: merge diffcore_pickaxe_grep() and diffcore_pickaxe_count() into diffcore_pickaxe()

diffcore_pickaxe_count() initializes the regular expression or kwset for
the search term, calls pickaxe() with the callback has_changes() and
cleans up afterwards.  diffcore_pickaxe_grep() does the same, only it
doesn't support kwset and uses the callback diff_grep() instead.  Merge
the two functions to form the new diffcore_pickaxe() and thus get rid of
the duplicate regex setup and cleanup code.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>