templater: add context.preload(t) to test if the specified template exists
I'm going to remove 'templ' from the resources dict because it is the only
resource that the caller can't provide. This also implies that putting
'templ' into the resources dict creates a reference cycle.
context.preload(t) will be used in place of templater.__contains__().
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 offset length ..... linkrev nodeid p1 p2 (re)
0 0 3 ..... 0 1e88685f5dde 000000000000 000000000000 (re)