tests: add test case for issue 6262
If you split a commit, prune one of the sides, and then try to revive
that commit by adding a successor, it will incorrectly be considered
divergence.
Differential Revision: https://phab.mercurial-scm.org/D10916
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true