I'm not a skilled UI designer, so I suspect there may be even better
options, but it's definitely already better than the raw ASCII "---" we
had before.
Put the line only at the beginning because it looks bad if the line
after the label is misaligned when labels don't have the same width
(e.g. "Remote" vs. "Local" in the divergence view).
Some tests assert that a specific commit subject does or doesn't occur
in the main view; interactive_rebase/outside_rebase_range_select.go is
an example for this, it asserts `t.Views().Main().Content(
DoesNotContain("commit 06"))`. The problem with this kind of assertion
and our test commit naming scheme is that the diff view begins with a
"commit <hash>" line, and when that hash happens to start with "06" the
assertion matched it and failed spuriously. This was usually masked by
our MaxAttempts=2 that we currently use for integration tests (it's
quite unlikely that the commit gets a hash beginning with "06" twice in
a row). However, we want to get to a state where we can set MaxAttempts
to 1, so make this more robust by changing our naming scheme.
It starts a rebase on the bottom-most commit of the range, and sets all the
selected commits to "edit" (skipping merge commits, because they can't be
edited).
