git grep shows the same hit repeatedly for unmerged paths

git grep shows the same hit repeatedly for unmerged paths

When the index is unmerged, e.g.

	$ git ls-files -u
        100644 faf413748eb6ccb15161a212156c5e348302b1b6 1	setup.c
        100644 145eca50f41d811c4c8fcb21ed2604e6b2971aba 2	setup.c
        100644 cb9558c49b6027bf225ba2a6154c4d2a52bcdbe2 3	setup.c

running "git grep" for work tree files repeats hits for each unmerged
stage.

	$ git grep -n -e setup_work_tree -- '*.[ch]'
        setup.c:209:void setup_work_tree(void)
        setup.c:209:void setup_work_tree(void)
        setup.c:209:void setup_work_tree(void)

This should fix it.

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