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
diff --git a/docs/manifest-format.txt b/docs/manifest-format.txt
index d5c6a02..1aa9396 100644
--- a/docs/manifest-format.txt
+++ b/docs/manifest-format.txt
@@ -244,7 +244,7 @@
 
 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.