If I arc patch a differential revision, arc land fails to land it:
```lang=none
chris@boltpro:~/git/puppet (master)$ arc patch D5846
Created and checked out branch arcpatch-D5846.
Checking patch modules/cruisecontrol/files/cruiseconduit/src/cruise/conduit/CruiseConduitClient.php...
Applied patch modules/cruisecontrol/files/cruiseconduit/src/cruise/conduit/CruiseConduitClient.php cleanly.
OKAY Successfully committed patch.
chris@boltpro:~/git/puppet (arcpatch-D5846)$ arc land
Landing current branch 'arcpatch-D5846'.
Switched to branch master. Updating branch...
The following commit(s) will be landed:
13ef8f4 Update the phpunit conduit client for the new report format refs T13573
Switched to branch arcpatch-D5846. Identifying and merging...
Usage Exception: arc can not identify which revision exists on branch 'arcpatch-D5846'. Update the revision with recent changes to synchronize the branch name and hashes, or use 'arc amend' to amend the commit message at HEAD, or use '--revision <id>' to select a revision explicitly.
```
I can run `arc land --revision D5846` but it would be great if arc knew the revision automatically.