strip: add a --keep test related to removing files from dirstate

When strip builds the list of changedfiles to pass into dirstate.rebuild, it adds
files blindly, including those that have been removed. This tests ensures that
rebuild can handle this case.

tests/test-strip.t

@@ -552,12 +552,19 b' Strip adds, removes, modifies with --kee'
   $ echo b > b
   $ echo d > d
   $ hg strip --keep tip
-  saved backup bundle to $TESTTMP/test/.hg/strip-backup/57e364c8a475-4cfed93c-backup.hg (glob)
+  saved backup bundle to $TESTTMP/test/.hg/strip-backup/*-backup.hg (glob)
   $ hg status
   M b
   ! bar
   ? c
   ? d
+
+... after updating the dirstate
+  $ hg add c
+  $ hg commit -mc
+  $ hg rm c
+  $ hg commit -mc
+  $ hg strip --keep '.^' -q
   $ cd ..
 
 stripping many nodes on a complex graph (issue3299)