Add context menus

[andree182]

For people that don't want to keep 20 key bindings in head, it could be very handy, if some kind of simple context menus (accessible e.g. via key 'x' :)) would be available - similar to what gitk has. It can be achieved using 'dialog' or similar, but it's not as seamless (+some functionality has to be reimplemented).

It could probably just show the currently applicable bindings, for simple implementation.

Example:

1 * [HEAD] [STABLE_123] {origin/STABLE_123} set release
2 * fix bug abc
3 | * [master] fix bug abc
4 | * {origin/master} commit xxx
5 |/
6 * commit yyy
7 * {origin/STABLE_42} commit zzz

On line 1:

• Allow choosing one of the refs (because %(refname) only picks the latest one, e.g. HEAD - and parsing %(text) is not so nice...). Perhaps such thing could also be accessed by %(chosen_ref) or something?
• Have a commit/branch/tag-predefined actions, so after picking the ref, a followup menu could do 'git checkout', 'git checkout -f', 'git rebase', forward it to clipboard or whatever else.

On line 4, options like:

• git cherry-pick %(ref)
• git reset --hard %(ref)

The context menus could be configured very similarly to the key bindings.