chgserver: make it a core module and drop extension flags
It was an extension just because there were several dependency cycles I
needed to address.
I don't add 'chgserver' to extensions._builtin since chgserver is considered
an internal extension so nobody should enable it by their config.
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b (glob)
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b (glob)
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..