diffcore-pickaxe: fix leaks in "log -S<block>" and "log -G<pattern>"

diffcore-pickaxe: fix leaks in "log -S<block>" and "log -G<pattern>"

The diff_grep() and has_changes() functions had early return
codepaths for unmerged filepairs, which simply returned 0.  When we
taught textconv filter to them, one was ignored and continued to
return early without freeing the result filtered by textconv, and
the other had a failed attempt to fix, which allowed the planned
return value 0 to be overwritten by a bogus call to contains().

Signed-off-by: Junio C Hamano <gitster@pobox.com>