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>

cgit.h

@@ -159,6 +159,7 @@ extern int chk_zero(int result, char *msg);
 extern int chk_positive(int result, char *msg);
 
 extern int hextoint(char c);
+extern char *trim_end(const char *str, char c);
 
 extern void *cgit_free_commitinfo(struct commitinfo *info);