keyword: use templatefilter to mark a function as template filter
This patch also adds test for filter 'svnisodate' and 'svnutcdate' for
safety, because there is no test using them, yet.
$ 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