"repo init --reference" can reuse existing repo tree. But chromeos sdk doesn't work with reference to mirror.

A) This is good:
  cd $tree1
  repo init ...skip...
  repo sync
  cd $tree2
  repo init ...skip... --reference=$tree1
  repo sync
  cros_sdk

B) this doesn't work:
  cd $tree1
  repo init ...skip... --mirror
  repo sync
  cd $tree2
  repo init ...skip... --reference=$tree1
  repo sync
  cros_sdk

When entering cros_sdk, the sdk will modify git's alternates information in order to make the git reference work both inside and outside chroot. However, the logic only support case A, not case B.

Detail analysis:
The underlying magic of "repo reference" is git's .git/objects/info/alternates. Take chromite as an example, 

A) good case:
After repo init and sync, the content of chromite/.git/objects/info/alternates is "/path/to/tree1/.repo/projects/chromite.git/objects", so git knows where to search git objects in additional to current folder.

In order to make repo reference work for both inside and outside chroot, the sdk will modify chromite's git alternates to somewhere inside .repo/alternates and bind mount .repo/alternates to .repo/chroot/alternates inside chroot. The final result of chromite's alternates is
/path/to/tree1/.repo/project-objects/chromiumos/chromite.git/objects (outside chroot)
/mnt/host/source/.repo/chroot/external1/.repo/project-objects/chromiumos/chromite.git/objects (inside chroot)
(sdk will bind mount $tree1 into .repo/chroot/external1 to make it visible inside chroot)

(some detail skipped since this is good case)

B) bad case:
After repo init and sync, the content of chromite/.git/objects/info/alternates is "/path/to/tree1/chromiumos/chromite.git/objects". This is correct because the tree layout of mirror is different to case A.

When entering chroot, it will execute 
    ./chromite/lib/rewrite_git_alternates.py $tree2 $tree1 /mnt/host/source 
In rewrite_git_alternates.py, _UpdateAlternatesDir()
68        suffix = os.path.join('.repo', 'project-objects', project, 'objects')
69        if os.path.exists(os.path.join(k, suffix)):
70          paths.append(os.path.join(v, suffix))

It searches git objdir in .repo/project-objects/$project/objects. However, ".repo/project-objects" is only created for normal repo checkout, not for repo mirror. 
Refers to https://chromium.googlesource.com/external/repo.git/+/master/manifest_xml.py#845
    if self.IsMirror:       
      ...
      gitdir = os.path.join(self.topdir, '%s.git' % name)                                                                                                                                                                                                                                                                 
      objdir = gitdir
    else:
      ...
      objdir = os.path.join(self.repodir, 'project-objects', '%s.git' % name)


==============
Given this issue existing several years, nobody use "repo reference to mirror" feature. This issue is worth to be fixed
 1. This benefits whom want to repo sync several chromeos tree, especially different milestones, versions, etc. (i.e. bisect)
 2. bisect-kit's chromeos bisector already uses "repo mirror" (to analyze full git history), so it's natural step for bisect-kit to use "reference to mirror". This issue need to be fixed or workaround.