strip: with --keep, consider all revs "removed" from the wcp (issue6270)

The current code was blink to change from other branches when stripping merges
that are ancestors of the working copy parents.

hgext/strip.py

@@ -254,7 +254,7 b' def stripcmd(ui, repo, *revs, **opts):'
 
             # only reset the dirstate for files that would actually change
             # between the working context and uctx
-            descendantrevs = repo.revs(b"%d::.", uctx.rev())
+            descendantrevs = repo.revs(b"only(., %d)", uctx.rev())
             changedfiles = []
             for rev in descendantrevs:
                 # blindly reset the files, regardless of what actually changed

tests/test-strip.t

@@ -1179,7 +1179,11 b' test stripping a working directory paren'
 
 stripping a set containing a merge properly reset file content, including items on other branches
 
-BROKEN, The added file should move to unknown, which is the behavior we have been seeing for other `hg strip --keep` call.
+The added file is moved to unknown, which is the behavior we have been seeing for other `hg strip --keep` call.
+
+stripping a set containing a merge properly reset file content, including items on other branches
+
+The added file is moved to unknown, which is the behavior we have been seeing for other `hg strip --keep` call.
 
   $ hg unbundle -u $TESTTMP/issue4736/.hg/strip-backup/35358f982181-a6f020aa-backup.hg
   adding changesets
@@ -1260,13 +1264,10 b' BROKEN, The added file should move to un'
      summary:     commitA
   
 
-BROKEN: 'bar.txt' cannot possibly be clean since it is not part of the current
-working copy parents.
-
   $ hg status -A
+  ? bar.txt
   C a
   C b
-  C bar.txt
   C foo.txt
   $ cat bar.txt
   bar