edit only for head revision otherwise open as tempfile

[extrawurst]

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)

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