Fix "grep -w"

Fix "grep -w"

We used to find the first match of the pattern and then if the
match is not for the entire word, declared that the whole line
does not match.

But that is wrong.  The command "git grep -w -e mmap" should
find that a line "foo_mmap bar mmap baz" matches, by tring the
second instance of pattern "mmap" on the same line.

Problems an earlier round of "fix" had were pointed out by Morten
Welinder, which have been incorporated in the t7002 tests.

Signed-off-by: Junio C Hamano <redacted>