test-dispatch: stabilize the test
When cwd is removed and `hg` is executed, some shells may run `getcwd`
before forking and executing, some may not do it, some may print a
different error message.
The test should be shell-independent so let's just avoid checking the error
message.
Differential Revision: https://phab.mercurial-scm.org/D1282
#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 ..