allow blob extract blobs by head/path combination

If blob is invoked with no id=, it tries to look up h= and search for path= in
 there. Once found, proceed as normal, otherwise, fail as normal.

Signed-off-by: Michael Krelin <hacker@klever.net>

cmd.c

@@ -32,7 +32,7 @@ static void about_fn(struct cgit_context *ctx)
 
 static void blob_fn(struct cgit_context *ctx)
 {
-	cgit_print_blob(ctx->qry.sha1, ctx->qry.path);
+	cgit_print_blob(ctx->qry.sha1, ctx->qry.path, ctx->qry.head);
 }
 
 static void commit_fn(struct cgit_context *ctx)