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.