update bash completion (like git's current)

[lbonn]

The bash completion script have been adapted from its counterpart in git.

However, it seems that it now lags a bit behind in terms of functionalities.

In particular, I often use tig to show the changes introduced by a branch: tig ..branch_example and being able to type tig ..br<tab> would be handful (this works with git log in current versions)

This may also apply to zsh for some other patterns but this particular use case seems to be covered.

[jonas]

Yes, I agree that it could need an overhaul.
Help with this would be really appreciated!

[lbonn]

I made a lousy attempt in the train last week but I didn't have a doc on bash completion's internals with me.

I'll try again if I have some free time in the next few weeks.

[rohieb]

This should now be solved by #960.