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 execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..