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
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