templatekw: add 'requires' flag to switch to exception-safe interface The current templatekw interface, f(repo, ctx, templ, **args), is horrible because it's quite easy to encounter TypeError, ValueError, etc. It's also bad for Python 3 porting due to the **kwargs issue. This patch introduces a flag to switch to new f(context, mapping) API seen in templater functions. The requirement spec isn't verified (yet) because context.resource() can gracefully raise a ResourceUnavailable exception, but it's planned to be used as a filter in the help, such as "Revision Keywords" (if 'ctx' in requires), "File Keywords" (if 'fctx' in requires), etc. showauthor() is ported to the new API as an example. 20 more follows.

templater: specialize ResourceUnavailable error so that it can be caught See the next patch how it will be used.

templater: move specialized exception types to top I'm going to add one more exception type.

templatekw: minimize resource dependency of {envvars} and {termwidth} These keywords can be evaluated without a repo.

templatekw: simply override {graphwidth} function by mapping variable

remotenames: drop redundant templatekw names from help text Also removed the second "List of" phrase, which is obvious from the type "List of strings."