template: add ifcontains template function
Adds a template function with the signature 'ifcontains(item, set, then[,
else])'. It can be used to do things like '{ifcontains('.hgignore',
file_mods, label(...), ...)}' to color commits that edit the .hgignore file.
A future patch will add the revset() function which will combine with
ifcontains to allow us to color commits if they are in the revset.
$ hg init
$ touch a
$ unset HGUSER
$ echo "[ui]" >> .hg/hgrc
$ echo "username= foo" >> .hg/hgrc
$ echo " bar1" >> .hg/hgrc
$ hg ci -Am m
adding a
abort: username 'foo\nbar1' contains a newline
[255]
$ rm .hg/hgrc
$ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
abort: username 'foo\nbar2' contains a newline
[255]
$ hg ci -Am m -u "`(echo foo; echo bar3)`"
transaction abort!
rollback completed
abort: username 'foo\nbar3' contains a newline!
[255]