test: preserve the existing module policy config for testrepohg resuse
The test runner is, righfully, setting HGMODULEPOLICY to match the flavor
requests to run the tests. However this mess with the `testrepohg` ability to
run properly, as the newly set policy might be incompatible with its
installation, or its ability to read the test repo.
So we preserve the initial value in a dedicated variable and use it in the
`testrepohg` helper.
#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 ..