templater: load template fragments from [templates] section in map file
This allows us to %include map-cmdline.<style> file in our .hgrc files. The
syntax is slightly different as hgrc doesn't support loading an external
template file, but map-cmdline files don't use this feature, so the syntax
can be considered identical in practice.
Unnamed section is remapped for backward compatibility.
%include map-cmdline.default
[templates]
changeset = '{cset}{branches}{bookmarks}{tags}{lphase}{parents}{user}{ldate}{summary}\n'
changeset_verbose = '{cset}{branches}{bookmarks}{tags}{lphase}{parents}{user}{ldate}{lfiles}{lfile_copies_switch}{description}\n'