obsolete: add a `commonversion` function
This function returns the highest common version between the locally known
formats and a list of remotely known formats. This is going to be useful to
know what format should be used for exchange.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a