Don't allow short names with .git suffix

Short names with .git suffix could under rare circumstances
lead to the git http-backend being invoked when a browser
was trying to view a regular commit.

Steps for obscure bug:
1. add repo short name "c" with source url:
   https://github.com/jensl/critic.git
2. add repo short name "c.git" with source url:
   https://github.com/jensl/v8-jsshell.git
3. go to "Branches", select "master", click the first commit
4. commit is not shown, web server sends no data back
Add TC for "Don't allow short names with .git suffix"
Merge remote-tracking branch 'github/master' into r/molsson/showcommit_sends_no_data

Conflicts:
	src/operation/addrepository.py
	testing/tests/001-main/002-createrepository.py