grep: NUL terminate input from a file

grep: NUL terminate input from a file

Internally "git grep" runs regexec(3) that expects its input string
to be NUL terminated.  When searching inside blob data, read_sha1_file()
automatically gives such a buffer, but builtin-grep.c forgot to put
the NUL at the end, even though it allocated enough space for it.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>