grep: plug a trivial memory leak

grep: plug a trivial memory leak

Change the cleanup phase for the grep command to free the pathspec
struct that's allocated earlier in the same block, and used just a few
lines earlier.

With "grep hi README.md" valgrind reports a loss of 239 bytes now,
down from 351.

The relevant --num-callers=40 --leak-check=full --show-leak-kinds=all
backtrace is:

    [...] 187 (112 direct, 75 indirect) bytes in 1 blocks are definitely lost in loss record 70 of 110
    [...]    at 0x4C2BBAF: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    [...]    by 0x60B339: do_xmalloc (wrapper.c:59)
    [...]    by 0x60B2F6: xmalloc (wrapper.c:86)
    [...]    by 0x576B37: parse_pathspec (pathspec.c:652)
    [...]    by 0x4519F0: cmd_grep (grep.c:1215)
    [...]    by 0x4062EF: run_builtin (git.c:371)
    [...]    by 0x40544D: handle_builtin (git.c:572)
    [...]    by 0x4060A2: run_argv (git.c:624)
    [...]    by 0x4051C6: cmd_main (git.c:701)
    [...]    by 0x4C5901: main (common-main.c:43)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>