git-grep: Fix problems with recently added tests

git-grep: Fix problems with recently added tests

Brian Gernhardt reported that test 'git grep -E -F -G a\\+b' fails on
OS X 10.6.7. This is because I assumed \+ is part of BRE, which isn't
true on all platforms.

The easiest way to make this test pass is to just update expected
output, but that would make the test pointless. Its real purpose is to
check whether 'git grep -E -F -G' is different from 'git grep -E -G -F'.
To check that, let's change pattern to "a+b*c". This should return
different match for -G, -F and -E.

I also made two small tweaks to the tests. First, I added path "ab" to
all calls to future-proof tests. Second, I updated last two tests to
better show that 'git grep -P -E' is different from 'git grep -E -P'.

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>