Add cgit_commit_link() + support for id=sha1 to commit view

This adds a function to generate links to the commit page and extends said
page to use id from querystring as primary revision specified (fallback to
h).

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

cgit.h

@@ -205,6 +205,8 @@ extern void cgit_tree_link(char *name, char *title, char *class, char *head,
 			   char *rev, char *path);
 extern void cgit_log_link(char *name, char *title, char *class, char *head,
 			  char *rev, char *path);
+extern void cgit_commit_link(char *name, char *title, char *class, char *head,
+			     char *rev);
 
 extern void cgit_print_error(char *msg);
 extern void cgit_print_date(time_t secs, char *format);