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 <ks1322@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/pathspec.c b/pathspec.c
index 7f88f1c..bb1efe1 100644
--- a/pathspec.c
+++ b/pathspec.c
@@ -467,7 +467,12 @@ static void init_pathspec_item(struct pathspec_item *item, unsigned flags,
 		match = prefix_path_gently(prefix, prefixlen,
 					   &prefixlen, copyfrom);
 		if (!match) {
-			const char *hint_path = get_git_work_tree();
+			const char *hint_path;
+
+			if (!have_git_dir())
+				die(_("'%s' is outside the directory tree"),
+				    copyfrom);
+			hint_path = get_git_work_tree();
 			if (!hint_path)
 				hint_path = get_git_dir();
 			die(_("%s: '%s' is outside repository at '%s'"), elt,