Commit 'aliases'

Description

Somebody added and then deleted a large file in two separate commits, which were not squashed during the merge request (review only tracked the "final" changeset, which didn't show the file in question). This went unnoticed (it took someone with a slower connection to complain about the slow fetching/cloning) for a few weeks, and many commits were added since.

We're OK to rewrite the history, but we have may links (within GitLab, emails, JIRA issues, etc.) that would become invalidated by this. Because GitLab is such a useful tool for looking at history and understand large projects, it would be nice to have commit 'aliases'.

Proposal

It would be great to have the possibility to define 'aliases' for commits, which would work as follows

1. They would be set up in the project page (i.e. not requiring GitLab admin rights -- Master (preferred) or Owner should be enough
2. If the reference to a HASH is not available in GitLab (e.g. because it was gc'd), then the route for the 'alias' hash should be shown, with a coloured warning box at the top of the page mentioning that this aliasing happened, but that the original hash is gone
3. If the reference to a HASH is available in GitLab, the original should be shown, and a coloured warning box at the top of the page mentioning that an new 'alias' commit exists
4. Of course this feature should remain optional

I post this in CE, but if you think it's more suited for EE, please move this accordingly.