backout: deprecate/hide support for backing out merges
This has never worked usefully:
- it can't undo a completely unwanted merge, as it leaves the merge in the DAG
- it can't undo a faulty merge as that means doing a merge correctly,
not simply reverting to one or the other parent
Both of these kinds of merge also require coordinated action among
developers to avoid the bad merge continuing to affect future merges,
so we should stop pretending that backout is of any help here.
As backing out a merge now requires a hidden option, it can't be done
by accident, but will continue to 'work' for anyone who's already
dependent on --parent for some unknown reason.
$ hg init a
$ cd a
$ echo 123 > a
$ hg add a
$ hg commit -m "a" -u a
$ cd ..
$ hg init b
$ cd b
$ echo 321 > b
$ hg add b
$ hg commit -m "b" -u b
$ hg pull ../a
pulling from ../a
searching for changes
abort: repository is unrelated
[255]
$ hg pull -f ../a
pulling from ../a
searching for changes
warning: repository is unrelated
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg heads
changeset: 1:9a79c33a9db3
tag: tip
parent: -1:000000000000
user: a
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
changeset: 0:01f8062b2de5
user: b
date: Thu Jan 01 00:00:00 1970 +0000
summary: b