Signed-off-by: Tamal Saha <tamal@appscode.com>

* Clean up ref name rules

Clean up checks on reference names to better conform to the guideline
here: https://git-scm.com/docs/git-check-ref-format

This fixes half of #6321

* Update branch create integration test

According to: https://git-scm.com/docs/git-check-ref-format

And: git check-ref-format "master/feature=test1"

This is a valid branch name and we should not be testing for it to fail.

* Add correct git branch name validation

* Change git refname validation error constant name

* Implement URL validation based on GoLang url.Parse method

* Backward compatibility with older Go compiler

* Add git reference name validation unit tests

* Remove unused variable in unit test

* Implement URL validation based on GoLang url.Parse method

* Backward compatibility with older Go compiler

* Add url validation unit tests