tests/test-issue842

templatefilters: add new stripdir filter

Adds a new template filter for removing directory levels from a
string. Examples:

{foo|stripdir} -> 'foo'
{foo/bar|stripdir} -> 'foo'
{foo/bar/more|stripdir} -> 'foo/bar'
{foo/bar/more|stripdir|stripdir} -> 'foo'

hg init test
cd test
echo foo > a
hg ci -Ama

hg up -r0000
echo bar > a
echo % should issue warning
hg ci -Amb

hg up -r0000
echo stuffy > a
echo % should not issue warning
hg ci -q -Amc

hg up -r0000
echo crap > a
hg branch testing
echo % should not issue warning
hg ci -q -Amd