Handle %xx encoding in querystring

Convert valid %xx expressions in querystring to ascii, ignore invalid
expressions (i.e. eat the three characters %xx).

Signed-off-by: Lars Hjemli <larsh@hal-2004.(none)>

cgit.h

@@ -67,6 +67,8 @@ extern void cgit_global_config_cb(const char *name, const char *value);
 extern void cgit_repo_config_cb(const char *name, const char *value);
 extern void cgit_querystring_cb(const char *name, const char *value);
 
+extern int hextoint(char c);
+
 extern void *cgit_free_commitinfo(struct commitinfo *info);
 
 extern char *fmt(const char *format,...);