Add a new warning during fetching git repo

Summary:
Improved the warning message showed during fetching a git repo if the specified folder is empty.

BUG: 368311
FIXED-IN: 5.2.0

Test Plan: Tested working for all possible cases while fetching a repo from git or Bazaar.

Reviewers: kfunk, #kdevelop

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3970