Previously, `doom upgrade` would emit:
```
New revision: fatal: amb (fatal: ambiguous argument '_upgrade/HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
```
It appears it's not possible to remote revisions with
`<remote>/<revision>`. Instead, we fetch the upgrade remote into a named
revision and then use it.
Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
bf50149ee3