grep: die gracefully when outside repository

grep: die gracefully when outside repository

Die gracefully when `git grep --no-index` is run outside of a Git
repository and the path is outside the directory tree.

If you are not in a Git repository and say:

    git grep --no-index search ..

You trigger a `BUG`:

    BUG: environment.c:213: git environment hasn't been setup
    Aborted (core dumped)

Because `..` is a valid path which is treated as a pathspec. Then
`pathspec` figures out that it is not in the current directory tree. The
`BUG` is triggered when `pathspec` tries to advise the user about how the
path is not in the current (non-existing) repository.

Reported-by: ks1322 ks1322 <redacted>
Helped-by: Junio C Hamano <redacted>
Signed-off-by: Kristoffer Haugsbakk <redacted>
Signed-off-by: Junio C Hamano <redacted>