templater: teach template loader to use open_template() function
The template loader can apparently load templates from relative paths,
so I needed to add that support to `open_template()`.
This takes the number of failing tests with PyOxidizer from 54 to 34.
Differential Revision: https://phab.mercurial-scm.org/D8907
#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 ..