highlight: adjust to attribute being private Differential Revision: https://phab.mercurial-scm.org/D3772

annotate: automatically populate fields referenced from template If '{line_number}' is in the template, we'll probably need it. No --line-number option should be required in such cases.

annotate: reverse mapping between option name and field name This makes the next patch slightly simpler.

formatter: provide hint of referenced field names I don't like the function name, but I call it datahint() for consistency with another function I'll add later, fm.contexthint().

templater: add function to look up symbols used in template Formatter can use this information to enable slow paths such as loading ctx object only when necessary.

templater: parse template string to tree by templater class The parsed tree could be cached, but it isn't for now. We can add a cache later if that matters.