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.

templater: drop extension point of engine classes (API) I don't think this would ever be used by third-party extensions, as we've heavily changed both the templater internals and the syntax since then. The main reason of removing this API is that I want to move the parsing function from the engine to the templater class so that we can peek keywords and functions used in a user template. This change also removes reference cycle between the templater and the engine.

templater: make it clearer that parsing doesn't cause recursion Only compileexp() may recurse into _load().

templater: mark most attributes as private

setdiscovery: use progress helper Differential Revision: https://phab.mercurial-scm.org/D3770

streamclone: use progress helper Differential Revision: https://phab.mercurial-scm.org/D3769

similar: use progress helper Note that a functional change here is that we now show the progress position *before* we start working on the item. This is consistent with how we do it elsewhere. Differential Revision: https://phab.mercurial-scm.org/D3768

remove: use progress helper Differential Revision: https://phab.mercurial-scm.org/D3767

changegroup: use progress helper in apply() (API) Differential Revision: https://phab.mercurial-scm.org/D3766