upgrade: register compression as a format variants
Compression is a promising vector for speedup, let us make it easier to check
the compression used and upgrade existing repository.
$ cat >> $HGRCPATH << EOF
> [ui]
> logtemplate="{rev}:{node|short} ({phase}) [{tags} {bookmarks}] {desc|firstline}\n"
> [extensions]
> dirstateparanoidcheck = $TESTDIR/../contrib/dirstatenonnormalcheck.py
> [experimental]
> nonnormalparanoidcheck = True
> [devel]
> all-warnings=True
> EOF
$ mkcommit() {
> echo "$1" > "$1"
> hg add "$1"
> hg ci -m "add $1"
> }
$ hg init testrepo
$ cd testrepo
$ mkcommit a
$ mkcommit b
$ mkcommit c
$ hg status