Fix moving a commit across a branch boundary in a stack of branches

[stefanhaller]

• PR Description

When moving the first commit of the second branch in a stack down by one (across the branch head of the first branch), the current behavior is broken: we move the commit only past the update-ref todo of branch1, which means the order of commits stays the same and only the branch head icon moves up by one. However, we move the selection down by one, so the wrong commit is selected now. This is especially bad if you type a bunch of ctrl-j quickly in a row, because now you are moving the wrong commit.

There would be two possible ways to fix this:

1. keep the moving behavior the same, but don't change the selection
2. change the behavior so that we move the commit not only past the update-ref, but also past the next real commit.

You could argue that 1) is the more desirable fix, as it gives you more control over where exactly the moved commit goes; however, it is much trickier to implement, so we go with 2) for now. If users need more fine-grained control, they can always enter an interactive rebase first.

While we're at it, also fix moving a commit across an exec todo in an interactive rebase, which is currently broken (it moves the commit one too far).

Fixes #4040.

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[stefanhaller]

This sits on top of #4094 right now, because it would otherwise conflict with some of its changes.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 75121381	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7512138)	Report Missing	Report Missing	Report Missing
Head commit (a9ef69b)	51356	44338	86.33%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4096)	52	52	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[jesseduffield]

nice work