string_list: add a function string_list_longest_prefix()

Add a function that finds the longest string from a string_list that
is a prefix of a given string.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-string-list.c

@@ -97,6 +97,26 @@ int main(int argc, char **argv)
 		return 0;
 	}
 
+	if (argc == 4 && !strcmp(argv[1], "longest_prefix")) {
+		/* arguments: <colon-separated-prefixes>|- <string> */
+		struct string_list prefixes = STRING_LIST_INIT_DUP;
+		int retval;
+		const char *prefix_string = argv[2];
+		const char *string = argv[3];
+		const char *match;
+
+		parse_string_list(&prefixes, prefix_string);
+		match = string_list_longest_prefix(&prefixes, string);
+		if (match) {
+			printf("%s\n", match);
+			retval = 0;
+		}
+		else
+			retval = 1;
+		string_list_clear(&prefixes, 0);
+		return retval;
+	}
+
 	fprintf(stderr, "%s: unknown function name: %s\n", argv[0],
 		argv[1] ? argv[1] : "(there was none)");
 	return 1;