rebase: delete obsolete internal "keepopen" option
The option was apparently introduced for use by the "pbranch"
extension, see
f2558a8228be (rebase: add option to not commit after a
collapsing, 2010-02-07). However, it doesn't seem like it was ever
used by that extension (according to `hg grep` in a clone of
https://bitbucket.org/parren/hg-pbranch/), so let's delete it.
Differential Revision: https://phab.mercurial-scm.org/D2708
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a