Add plain patch view

The new view mimics the output from `git format-patch`, making it possible
to cherry-pick directly from cgit with something like `curl $url | git am`.

Inspired by a patch to `git-apply` by Mike Hommey:
  http://thread.gmane.org/gmane.comp.version-control.git/67611/focus=67610

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

shared.c

@@ -70,7 +70,7 @@ int htmlfd = 0;
 int cgit_get_cmd_index(const char *cmd)
 {
 	static char *cmds[] = {"log", "commit", "diff", "tree", "blob",
-			       "snapshot", "tag", "refs", NULL};
+			       "snapshot", "tag", "refs", "patch", NULL};
 	int i;
 
 	for(i = 0; cmds[i]; i++)