grep: accept relative paths outside current working directory

"git grep" would barf at relative paths pointing outside the current
working directory (or subdirectories thereof). Use quote_path_relative(),
which can handle such cases just fine.

[jc: added tests.]

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.h

@@ -59,6 +59,7 @@ struct grep_opt {
 	struct grep_pat *pattern_list;
 	struct grep_pat **pattern_tail;
 	struct grep_expr *pattern_expression;
+	const char *prefix;
 	int prefix_length;
 	regex_t regexp;
 	unsigned linenum:1;