project: fix m/ pseudo ref handling with git worktrees

Since most ref namespaces are shared among all worktrees, trying to
set the pseudo m/<branch> in the common git repo ends up clobbering
each other when using shared checkouts.  For example, in CrOS:
  <project path="src/third_party/kernel/v3.8"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/chromeos-3.8" />
  <project path="src/third_party/kernel/v3.10"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/chromeos-3.10" />

Trying to set m/master in chromiumos/third_party/kernel.git/ will
keep clobbering the other.

Instead, when using git worktrees, lets set the m/ pseudo ref to
point into the refs/worktree/ namespace which is unique to each
git worktree.  So we have in the common dir:
  chromiumos/third_party/kernel.git/:
    refs/remotes/m/master:
      ref: refs/worktree/m/master
And then in each worktree we point refs/worktree/m/master to the
respective manifest revision expression.  Now people can use the
m/master in each git worktree and have it resolve to the right
commit for that worktree.

Bug: https://crbug.com/gerrit/12404
Change-Id: I78814bdd5dd67bb13218c4c6ccd64f8a15dd0a52
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256952
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

git_refs.py

@@ -23,6 +23,8 @@ R_CHANGES = 'refs/changes/'
 R_HEADS = 'refs/heads/'
 R_TAGS = 'refs/tags/'
 R_PUB = 'refs/published/'
+R_WORKTREE = 'refs/worktree/'
+R_WORKTREE_M = R_WORKTREE + 'm/'
 R_M = 'refs/remotes/m/'