templater: pass opened file-like object to templatespec
I think I said earlier that I planned to create a special templatespec
variant for built-in templates. That was true (I planned that), but I
ended up (in this patch) just adding a file-like object to the
`mapfile_templatespec()` variant instead.
Differential Revision: https://phab.mercurial-scm.org/D8893
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).