Add trim_end() and use it to remove trailing slashes from repo paths

The new function removes all trailing instances of an arbitrary character
from a copy of the supplied char array. This is then used to remove any
trailing slashes from cgit_query_path.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

parsing.c

@@ -168,7 +168,7 @@ void cgit_parse_url(const char *url)
 		if (p) {
 			p[0] = '\0';
 			if (p[1])
-				cgit_query_path = xstrdup(p + 1);
+				cgit_query_path = trim_end(p + 1, '/');
 		}
 		cgit_cmd = cgit_get_cmd_index(cmd + 1);
 		cgit_query_page = xstrdup(cmd + 1);