Fix `morph edit` when repo has the same ref as system branch

There was a check in it to see whether it needed to do the git branch
and git checkout based on whether the name of the branch matched that in
the morphology.

This had a couple of problems:

1.  Now that we aren't always building from HEAD, we need to be able to
    roll its commit back, so using the existing branch isn't always the
    best idea.
2.  It only checks the "ref" field, not "unpetrify-ref", so even though
    we clone the right ref in there, it's checking the commit id against
    the system branch name, so would always try to re-create the branch,
    and fail when it already exists.

So now, we remove the original ref and re-create it with our preferred
HEAD.

A better solution might be to change the clone logic to not
automatically checkout HEAD, and instead require an explicit branch then
checkout, but the initial clone logic is shared with build, and I didn't
feel like tracking down all the different places that it was used.

3 files changed, 32 insertions, 6 deletions

diff --git a/morphlib/gitdir.py b/morphlib/gitdir.py
index 3966a0f0..fea26c2e 100644
--- a/morphlib/gitdir.py
+++ b/morphlib/gitdir.py
@@ -495,8 +495,17 @@ class GitDirectory(object):
             raise InvalidRefError(self, ref)
 
     def disambiguate_ref(self, ref): # pragma: no cover
-        out = self._runcmd(['git', 'rev-parse', '--symbolic-full-name', ref])
-        return out.strip()
+        try:
+            out = self._runcmd(['git', 'rev-parse', '--symbolic-full-name',
+                                ref])
+            return out.strip()
+        except cliapp.AppException: # ref not found
+            if ref.startswith('refs/heads/'):
+                return ref
+            elif ref.startswith('heads/'):
+                return 'refs/' + ref
+            else:
+                return 'refs/heads/' + ref
 
     def resolve_ref_to_commit(self, ref):
         return self._rev_parse('%s^{commit}' % ref)
diff --git a/morphlib/plugins/branch_and_merge_plugin.py b/morphlib/plugins/branch_and_merge_plugin.py
index 434ff6af..36f720aa 100644
--- a/morphlib/plugins/branch_and_merge_plugin.py
+++ b/morphlib/plugins/branch_and_merge_plugin.py
@@ -309,9 +309,17 @@ class BranchAndMergePlugin(cliapp.Plugin):
                 cached_repo = lrc.get_updated_repo(chunk_url)
 
                 gd = sb.clone_cached_repo(cached_repo, chunk_ref)
-                if chunk_ref != sb.system_branch_name:
-                    gd.branch(sb.system_branch_name, chunk_ref)
-                    gd.checkout(sb.system_branch_name)
+                system_branch_ref = gd.disambiguate_ref(sb.system_branch_name)
+                sha1 = gd.resolve_ref_to_commit(chunk_ref)
+
+                try:
+                    old_sha1 = gd.resolve_ref_to_commit(system_branch_ref)
+                except morphlib.gitdir.InvalidRefError as e:
+                    pass
+                else:
+                    gd.delete_ref(system_branch_ref, old_sha1)
+                gd.branch(sb.system_branch_name, sha1)
+                gd.checkout(sb.system_branch_name)
                 gd.update_submodules(self.app)
                 gd.update_remotes()
                 if gd.has_fat():
diff --git a/yarns/branches-workspaces.yarn b/yarns/branches-workspaces.yarn
index 25ea7365..7c85d6e0 100644
--- a/yarns/branches-workspaces.yarn
+++ b/yarns/branches-workspaces.yarn
@@ -39,7 +39,7 @@ existing workspace, initialising it should fail.
     WHEN        the user attempts to initialise a workspace
     THEN        morph failed
 
-Checking out or branching system branches
+Checking out system branches
 -----------------------------------------
 
 Once we have a workspace, we can check out a system branch.
@@ -50,6 +50,12 @@ Once we have a workspace, we can check out a system branch.
     WHEN        the user checks out the system branch called master
     THEN        the system branch master is checked out
 
+Edit is probably not the best name for is, but  we can use `morph edit`
+to investigate chunks in existing branches.
+
+    WHEN the user edits the chunk test-chunk in branch master
+    THEN the edited chunk test:test-chunk has git branch master
+
 Checking out a system branch should fail, if the branch doesn't exist.
 
     SCENARIO    checking out a system branch that doesn't exist
@@ -58,6 +64,9 @@ Checking out a system branch should fail, if the branch doesn't exist.
     WHEN        the user attempts to check out the system branch called foo
     THEN        morph failed
 
+Branching system branches
+-----------------------------------------
+
 We can, instead, create a new system branch, off master.
 
     SCENARIO    branch off master