pushkey: splits obsolete marker exchange into multiple keys
Obsolete markers are now exchanged in smaller pieces that fit in a http header.
This changes is done to avoid 400 bad request error when exchanging obsolete
puskey over http.
The last key pushed is always hold by the "dump0" key to ensure an easy place to
hook for people who need it.
$ 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