tests: clarify test setup test in test-uncommit.t

I assume the "hg uncommit b" is there to prove that the working copy
is dirty before we try "hg uncommit --allow-dirty-working-copy b". It
seems clearer to put that check just before we run the actual test.

Differential Revision: https://phab.mercurial-scm.org/D6078

tests/test-uncommit.t

@@ -482,10 +482,6 b' Copy a->b1 and a->b2, then rename b1->c '
   A b
   $ hg unc a
   note: keeping empty commit
-  $ hg unc b
-  abort: uncommitted changes
-  (requires --allow-dirty-working-copy to uncommit)
-  [255]
   $ cat a
   super critical info!
   $ hg log
@@ -498,6 +494,10 b' Copy a->b1 and a->b2, then rename b1->c '
   
   $ hg ci -Am 'add b'
   $ echo 'foo bar' > b
+  $ hg unc b
+  abort: uncommitted changes
+  (requires --allow-dirty-working-copy to uncommit)
+  [255]
   $ hg unc --allow-dirty-working-copy b
   $ hg log
   changeset:   3:30fa958635b2