tests: add test of uncommit with default phase as secret

We didn't seem to have any test checking that uncommitting a draft
commit with phase.new-phase=secret preserved the draft phase.

Differential Revision: https://phab.mercurial-scm.org/D3817

tests/test-uncommit.t

@@ -278,6 +278,13 b' Uncommit with draft parent'
   12: draft
   $ hg commit -m 'update ab again'
 
+Phase is preserved
+
+  $ hg uncommit --keep --config phases.new-commit=secret
+  $ hg phase -r .
+  15: draft
+  $ hg commit --amend -m 'update ab again'
+
 Uncommit with public parent
 
   $ hg phase -p "::.^"
@@ -294,7 +301,7 b' Partial uncommit with public parent'
   $ hg status
   A xyz
   $ hg phase -r .
-  16: draft
+  18: draft
   $ hg phase -r ".^"
   12: public