chgserver: implement validate command
validate will load the repo config and check if the server has up-to-date
config to continue serve the client. In case it does not, the server will
send instructions to the client about what to do next, including to retry
with a different address or to unlink an outdated socket file to stop an
old server.
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 ..