3 files changed, 21 insertions, 12 deletions

diff --git a/git_config.py b/git_config.py
index e00f6be2..8c247394 100644
--- a/git_config.py
+++ b/git_config.py
@@ -50,16 +50,24 @@ else:
 from git_command import GitCommand
 from git_command import ssh_sock
 from git_command import terminate_ssh_clients
+from git_refs import R_CHANGES, R_HEADS, R_TAGS
 
-R_HEADS = 'refs/heads/'
-R_TAGS  = 'refs/tags/'
 ID_RE = re.compile(r'^[0-9a-f]{40}$')
 
 REVIEW_CACHE = dict()
 
+def IsChange(rev):
+  return rev.startswith(R_CHANGES)
+
 def IsId(rev):
   return ID_RE.match(rev)
 
+def IsTag(rev):
+  return rev.startswith(R_TAGS)
+
+def IsImmutable(rev):
+    return IsChange(rev) or IsId(rev) or IsTag(rev)
+
 def _key(name):
   parts = name.split('.')
   if len(parts) < 2:
diff --git a/git_refs.py b/git_refs.py
index 3c266061..58c838a6 100644
--- a/git_refs.py
+++ b/git_refs.py
@@ -16,11 +16,12 @@
 import os
 from trace import Trace
 
-HEAD    = 'HEAD'
-R_HEADS = 'refs/heads/'
-R_TAGS  = 'refs/tags/'
-R_PUB   = 'refs/published/'
-R_M     = 'refs/remotes/m/'
+HEAD      = 'HEAD'
+R_CHANGES = 'refs/changes/'
+R_HEADS   = 'refs/heads/'
+R_TAGS    = 'refs/tags/'
+R_PUB     = 'refs/published/'
+R_M       = 'refs/remotes/m/'
 
 
 class GitRefs(object):
diff --git a/subcmds/start.py b/subcmds/start.py
index 290b6897..c3ec303e 100644
--- a/subcmds/start.py
+++ b/subcmds/start.py
@@ -18,7 +18,7 @@ import os
 import sys
 
 from command import Command
-from git_config import IsId
+from git_config import IsImmutable
 from git_command import git
 import gitc_utils
 from progress import Progress
@@ -96,11 +96,11 @@ revision specified in the manifest.
           project.Sync_LocalHalf(sync_buf)
           project.revisionId = gitc_project.old_revision
 
-      # If the current revision is a specific SHA1 then we can't push back
-      # to it; so substitute with dest_branch if defined, or with manifest
-      # default revision instead.
+      # If the current revision is immutable, such as a SHA1, a tag or
+      # a change, then we can't push back to it. Substitute with
+      # dest_branch, if defined; or with manifest default revision instead.
       branch_merge = ''
-      if IsId(project.revisionExpr):
+      if IsImmutable(project.revisionExpr):
         if project.dest_branch:
           branch_merge = project.dest_branch
         else: