unshelve: add -k as short form of --keep

For parity with strip -k, rebase -k, etc.

hgext/shelve.py

@@ -572,7 +572,7 b' def unshelvecontinue(ui, repo, state, op'
            _('abort an incomplete unshelve operation')),
           ('c', 'continue', None,
            _('continue an incomplete unshelve operation')),
-          ('', 'keep', None,
+          ('k', 'keep', None,
            _('keep shelve after unshelving')),
           ('', 'date', '',
            _('set date for temporary commits (DEPRECATED)'), _('DATE'))],

tests/test-shelve.t

@@ -493,7 +493,7 b' test keep and cleanup'
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg shelve --list
   default         (*)    changes to 'create conflict' (glob)
-  $ hg unshelve --keep
+  $ hg unshelve -k
   unshelving change 'default'
   $ hg shelve --list
   default         (*)    changes to 'create conflict' (glob)