context: make changectx.mutable() benefit from .phase() logic This fixes "hg qimport -r null". Previous versions used to: - Traceback because null revision mutability was not defined - Add an empty -1.diff patch to the series The error message: abort: revision -1 is not mutable is symptomatic of a deeper problem in phase command revision handling. It could be fixed easily in the command itself but I feel a better fix must be done in phase API which raises the issue of phase updates atomicity: aborting in phases.advanceboundary/retractboundary requires a better rollback behaviour to avoid partial changes.

--- a/tests/test-mq-qimport.t	Mon Apr 02 22:25:55 2012 +0100
+++ b/tests/test-mq-qimport.t	Thu Apr 05 19:15:23 2012 +0200
@@ -28,6 +28,14 @@
   abort: unable to read file non-existing-file
   [255]
 
+qimport null revision
+
+  $ hg qimport -r null
+  abort: revision -1 is not mutable
+  (see "hg help phases" for details)
+  [255]
+  $ hg qseries
+
 import email
 
   $ hg qimport --push -n email - <<EOF