Started Mercurial backend implementation

Issue #511

Signed-off-by: Michal Čihař <michal@cihar.com>
Clarify that this is git repo

Signed-off-by: Michal Čihař <michal@cihar.com>
Prepare infrastructure for Mercurial repository testing

Issue #511

Signed-off-by: Michal Čihař <michal@cihar.com>
Basic test framework for Mercurial repository

Issue #511

Signed-off-by: Michal Čihař <michal@cihar.com>