rust: Add a Timestamp struct instead of abusing Duration
`SystemTime` would be the standard library type semantically appropriate
instead of `Duration`.
But since the value is coming from Python as a plain integer and used in
dirstate packing code as an integer, let’s make a type that contains a single
integer instead of using one with sub-second precision.
Differential Revision: https://phab.mercurial-scm.org/D10485
Issue351: mq: qrefresh can create extra revlog entry
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ hg qinit
$ echo b > b
$ hg ci -A -m foo
adding b
$ echo cc > b
$ hg qnew -f foo.diff
$ echo b > b
$ hg qrefresh
$ hg debugindex b
rev linkrev nodeid p1 p2
0 0 1e88685f5dde 000000000000 000000000000