Simplify nearest_branch algorithm

[kaste]

Closes #1166

This is the 'cheap' solution mentioned in #1166. We just take the first relative. By that we go from minimal 4 git commands down to 1.

[randy3k]

It kind of makes sense to me.
@asfaltboy what do you think? You were the one who wrote it.

[stoivo]

I did't touch rebase much, so up to you guys

[kaste]

git show-branch also has sorting options. Maybe it is enough to fiddle with them if we run into problems. After all we only want a good candidate, as explained in the issue there are valid other candidates but then we must open a panel and ask the user.

[kaste]

I actually don't use the rebase dashboard at all but wanted to use the 'nearest_branch' algo for something else.

[asfaltboy]

I think @kaste is right, we had originally considered picking nearest neighbour and be done with it, but then we saw some issues with many similar forked branches (see discussion starting around this comment).

Apparently, we complicated matters trying to find an earlier common node. In hindsight, we probably should have introduced these two methods separately to have time to test. I have no objection to merging this fix, as it fixes some issues I've been seeing and from my (limited) testing seems to work in all scenarios mentioned.

[kaste]

So, let's try this on dev.