ui-snapshot: add dwimmery

When downloading a snapshot, the snapshot name will often contain the repo
name combined with a tag. This patch tries to exploit this so that the
correct revision is downloaded even if no specific revision is specified.

PS: this only occurs if neither 'h' nor 'id' is specified in the query-
string.

PPS: this also fixes a bug which occurs when trying to download a filename
with an unsupported suffix: it used to try to print an error message to
the user but failed since it didn't prepare the output properly.

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

cmd.c

@@ -106,7 +106,7 @@ static void snapshot_fn(struct cgit_context *ctx)
 {
 	cgit_print_snapshot(ctx->qry.head, ctx->qry.sha1,
 			    cgit_repobasename(ctx->repo->url), ctx->qry.path,
-			    ctx->repo->snapshots);
+			    ctx->repo->snapshots, ctx->qry.nohead);
 }
 
 static void summary_fn(struct cgit_context *ctx)