#000 - Fix for null message in health state.

Especially in cases of plugins, if the return value is null, then
ServerHealthState constructor fails.
Post-commit hook for mercurial

Follows along the same line as git. Moved GitUrlMatchers to be generic enough to be used by mercurial as well.
#000 - Missed writing test.
Merge pull request #767 from arvindsv/fix_health_state_null

#000 - Fix for null message in health state.
Merge pull request #781 from sachinsudheendra/mercurial_post_commit_hook

Post-commit hook for mercurial