Allow disabling of precise contentHash to csid mapping

FishEye may spend a lot of time matching git content hashes to revisions. This is necessary because git's history provides content hashes, not actual revision ids. It is time-consuming when a repository has many revisions on the same path which have the same content.

The fix for this issue provides a --Xenable-git-content-hash-resolving-heuristic command line argument which allows FishEye to guess the revision when several match, at the cost of sometimes having incorrect revision ids displayed on the File tab of the repository browse page.

See the screenshots below for details.