tests: on windows, run command explicitly in sh for working command substitution
On windows, os.system runs the command in cmd.exe, which does not know command
substitution. To let this work, run the command explicitly in sh.
This fixes the failure of this test on windows.
$ "$TESTDIR/hghave" execbit || exit 80
$ 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 ..