templatekw: add {size} keyword as an example of fctx-based keyword I'll add {status}, and I think some lfs keywords can be migrated to this. I'm not certain how many fctx-based keywords will be introduced into the global space, but if there are a couple more, we'll probably need to sort them out to the "File Keywords" section in the templater help. Until then, fctx keywords are hidden as experimental.

formatter: populate fctx from ctx and path value Tests will be added by the next patch.

formatter: factor out function that detects node change and document it This prepares for demand loading of ctx/fctx objects. With this change, 'revcache' is also recreated if 'node' value changes, which will be needed to support loading of ctx from (repo, node) pair.