tests: use full `uncommit` command name in tests

I'm about to add a `hg uncopy`, so the `hg unc` we used for `hg
uncommit` would become ambiguous.

Differential Revision: https://phab.mercurial-scm.org/D8028

tests/test-uncommit.t

@@ -489,7 +489,7 b' Copy a->b1 and a->b2, then rename b1->c '
   $ hg add b
   $ hg status
   A b
-  $ hg unc a
+  $ hg uncommit a
   note: keeping empty commit
   $ cat a
   super critical info!
@@ -503,11 +503,11 b' Copy a->b1 and a->b2, then rename b1->c '
   
   $ hg ci -Am 'add b'
   $ echo 'foo bar' > b
-  $ hg unc b
+  $ hg uncommit b
   abort: uncommitted changes
   (requires --allow-dirty-working-copy to uncommit)
   [255]
-  $ hg unc --allow-dirty-working-copy b
+  $ hg uncommit --allow-dirty-working-copy b
   $ hg log
   changeset:   3:30fa958635b2
   tag:         tip