don't try to interpret treepath as hash (#17272)

...when path contains no hash-path-separator ('/')

This is a workaround to #17179.

Entering this case when `path` does not contain a '/' does not really
make sense, as that means the tree path is empty, but this case is only
entered for routes that expect a non-empty tree path.

Treepaths like <40-char-dirname>/<filename> will still fail,
but hopefully don't occur that often. A more complete fix that avoids
this case too is outlined in #17185, but too big of a change to backport

modules/context/repo.go

@@ -695,7 +695,7 @@ func getRefName(ctx *Context, pathType RepoRefType) string {
 		}
 		// For legacy and API support only full commit sha
 		parts := strings.Split(path, "/")
-		if len(parts) > 0 && len(parts[0]) == 40 {
+		if len(parts) > 1 && len(parts[0]) == 40 {
 			ctx.Repo.TreePath = strings.Join(parts[1:], "/")
 			return parts[0]
 		}