edit only for head revision otherwise open as tempfile

right now we show a `edit [e]` cmd for any file at any revision in file tree views. but we only open the file as it is currently on disk.

this means you don't see then what you expected when you looked at an old diff/revision. or worst case: the file is no longer present and will error.

we should only show `edit [e]` when you are at the head revision and otherwise two other options:
1. `open [o]` which will dump the revision of the file in a tempfile and open that
2. `go to head [g]` to jump the that file in the current file-tree (if it present)

2. is not as trivial and should probably be a separate issue (feel free to open that)

Start in the file-tree view handling for the existing `edit [e]` command and trace how a selected revision is opened. Limit the change to showing edit only at the head revision and opening older revisions through the proposed `open [o]` tempfile behavior; treat `go to head [g]` as separate work. Done means old revisions open successfully without depending on the current on-disk file.

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