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
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: