Introduce rebase support

[simoneb]

Unless I'm missing something there doesn't seem to exist support for rebasing yet. It would be nice to have it.

[ethomson]

Nope, no rebase support yet. This depends on libgit2/libgit2#2253

[Aimeast]

+1

[sandyarmstrong]

See also libgit2/libgit2#1415

[Therzok]

There is rebase support implemented using merge.

[carlosmn]

Rebase was introduced in #964

[felixfbecker]

Could someone give me an example of function calls for one rebase?