extend manifest with keys to be deleted

This will eventually be used to recover from an interrupted fullPush
and drop the old bundle keys it was unable to delete.

Sponsored-by: Luke T. Shumaker on Patreon

doc/internals/git-remote-annex.mdwn

@@ -12,6 +12,10 @@ GITBUNDLE--$UUID-sha256 is a git bundle.
 
 An ordered list of bundle keys, one per line. 
 
+Additionally, there may be bundle keys that are prefixed with "-".
+These keys are not part of the current content of the git remote
+and are in the process of being deleted.
+
 (Lines end with unix `"\n"`, not `"\r\n"`.)
 
 # multiple GITMANIFEST files

doc/todo/git-remote-annex.mdwn

@@ -33,6 +33,9 @@ This is implememented and working. Remaining todo list for it:
   to clone from a wrong url, since it fails to download
   a manifest and so appears as if the remote is empty.
 
+* Improve recovery from interrupted push by using outManifest to clean up
+  after it. (Requires populating outManifest.)
+
 * See XXX in uploadManifest about recovering from a situation
   where the remote is left with a deleted manifest when a push
   is interrupted part way through. This should be recoverable