support conflict-free rebase instead of merge

[extrawurst]

(based on #302)

now that we support merging a branch #485 we can look into rebase instead of merge.
although this gets a tad more complicated since any rebase step could fail due to conflicts and then we need to be able to proceed the rebase after conflict resolution was done.

[IndianBoy42]

Related to this: the pull branch feature will currently always merge, even though I have set to rebase by default in my ~/.gitconfig. I hope this will be changed once rebasing is supported.

Is the work to be done on this mostly user interface related or some technical limitations? AFAICT, If we can see/fix merge conflicts in gitui currently, rebasing should be very similar

[extrawurst]

Is the work to be done on this mostly user interface related or some technical limitations?

like I stated above:

any rebase step could fail due to conflicts and then we need to be able to proceed the rebase after conflict resolution was done.

[extrawurst]

• basic implementation
• unittest