RepositoryService: WriteRef for special refs

Currently, WriteRef accepts only one case: the default branch (which is just HEAD in Git).

For heptapod#431 (closed), and to conclude the %HGitaly1: Hg SHAs milestone, leaving the data in good shape for the move to %HGitaly2: fully native HGitaly2, it needs to treat all special refs, including keep-arounds.

Edited Mar 09, 2021 by Georges Racinet

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information