Allow to view history for arbitrary commit

Right now I could use the following URL:

projects/PROJECT_1/repos/rep_1/commits?until=refs/heads...

to view branch history. There is no way to use non refs/heads branch or commitId as "until" value to view history starting from a particular commit or for a non-standard branch/tag.

Let user use commit SHA-1 as "until" parameter" or branch ref other than one in the refs/heads.

We need that, as new version of our SVN Mirror Add-On keeps track of unsynced commits by putting them into "refs/subgit/unsynced" namespace.

For instance, unsynced commit could appear when user decides to mirror non-empty Git repository with a Subversion project, then, if there are branches already present in both repositories, those on the Git side will be moved to "refs/subgit/unsynced" namespace. Later user is provided with the UI where it is possible to review such branches and merge them back to repository or discard. Same situation could appear when user disables mirror and them enables it back when histories are already diverged - Git side will be moved to "refs/unsynced" namespsace then.

For that to work smoothly we need two improvements - first is the one above (show history starting from a particular commit or from non refs/heads references) and second is to let pull request use arbitrary commit or non refs/heads branch as a source (this I'll put in a separate issue).