fix: add :enabled sub-config for fixer tools
This allows distributing opt-in fixer tool configurations in .hgrc
files. This may be useful for adding default configs in core, or for
orgranizations that want to provide configs to their users.
Tools are still enabled by default because it would be confusing to add a
config and find that it has no effect until you add enabled=true.
Differential Revision: https://phab.mercurial-scm.org/D6975
$ hg init repo
$ cd repo
$ touch foo
$ hg ci -Am 'add foo'
adding foo
$ hg up -C null
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
this should be stored as a delta against rev 0
$ echo foo bar baz > foo
$ hg ci -Am 'add foo again'
adding foo
created new head
$ hg debugindex foo
rev linkrev nodeid p1 p2
0 0 b80de5d13875 000000000000 000000000000
1 1 0376abec49b8 000000000000 000000000000
$ cd ..