Support specifying non-HEADS refs as upstream

While not typical, some users might have an upstream that isn't in
the usual refs/heads/* namespace. There's no reason not to use
those refs as the value for the upstream attribute, so support
doing so.

Change-Id: I5b119f1135c3268c20e7c4084682e860d3ee1fb1

docs/manifest-format.txt

@@ -244,7 +244,7 @@ whole ref space.
 
 Attribute `sync-s`: Set to true to also sync sub-projects.
 
-Attribute `upstream`: Name of the Git branch in which a sha1
+Attribute `upstream`: Name of the Git ref in which a sha1
 can be found.  Used when syncing a revision locked manifest in
 -c mode to avoid having to sync the entire ref space.
 

git_config.py

@@ -619,8 +619,6 @@ class Remote(object):
     """
     if IsId(rev):
       return rev
-    if rev.startswith(R_TAGS):
-      return rev
 
     if not rev.startswith('refs/'):
       rev = R_HEADS + rev
@@ -628,6 +626,10 @@ class Remote(object):
     for spec in self.fetch:
       if spec.SourceMatches(rev):
         return spec.MapSource(rev)
+
+    if not rev.startswith(R_HEADS):
+      return rev
+
     raise GitError('remote %s does not have %s' % (self.name, rev))
 
   def WritesTo(self, ref):

project.py

@@ -1730,10 +1730,11 @@ class Project(object):
     if depth:
       current_branch_only = True
 
-    if current_branch_only:
     if ID_RE.match(self.revisionExpr) is not None:
       is_sha1 = True
-      elif self.revisionExpr.startswith(R_TAGS):
+
+    if current_branch_only:
+      if self.revisionExpr.startswith(R_TAGS):
         # this is a tag and its sha1 value should never change
         tag_name = self.revisionExpr[len(R_TAGS):]
 
@@ -1820,9 +1821,10 @@ class Project(object):
       branch = self.revisionExpr
       if is_sha1:
         branch = self.upstream
-      if branch.startswith(R_HEADS):
-        branch = branch[len(R_HEADS):]
-      spec.append(str((u'+refs/heads/%s:' % branch) + remote.ToLocal('refs/heads/%s' % branch)))
+      if branch is not None and branch.strip():
+        if not branch.startswith('refs/'):
+          branch = R_HEADS + branch
+        spec.append(str((u'+%s:' % branch) + remote.ToLocal(branch)))
     cmd.extend(spec)
 
     shallowfetch = self.config.GetString('repo.shallowfetch')