obsolete: introduce an `_enabled` switch to disable the feature by default
Obsolete markers wide-usage and propagation should be avoided by default until
the obsolete feature is more mature.
This changeset introduce the `_enable` variable and prevent the creation of
obsolete marker if the feature is set to `False` (the default).
More limitation comes in followup changesets.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
committer test 0 0
revision 0
branch default
adda
$ cd ..