extension: add a `required` suboption to enforce the use of an extensions
If `required` is set, failing to load an extensions will abort. See the test
and documentation for details.
Differential Revision: https://phab.mercurial-scm.org/D11822
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls -A
.hg
a
unknown