phases: add a list of names usable by the phase command

This is useful in TortoiseHg for example, which has until recently just added
all names in the `phasenames` list to the context menu that changes the phase.
The ones not supported by the command would just error out.

mercurial/phases.py

@@ -133,8 +133,9 b' archived = HIDEABLE_FLAG'
 allphases = range(internal + 1)
 trackedphases = allphases[1:]
 # record phase names
+cmdphasenames = ['public', 'draft', 'secret']  # known to `hg phase` command
 phasenames = [None] * len(allphases)
-phasenames[:3] = ['public', 'draft', 'secret']
+phasenames[:len(cmdphasenames)] = cmdphasenames
 phasenames[archived] = 'archived'
 phasenames[internal] = 'internal'
 # record phase property