workingctx: correctly compute the flag for noexec filesystems+merge
This bug happens if the filesystem doesn't support exec-bit, during merges,
for example in 24ed7a541f23 on the hg repo.
If f is not in p1, but is in p2 and has the x-bit in p2, since the dirstate is
based on p1, and the FS doesn't support the exec-bit, the dirstate can't
"guess" the right bit.
We instead fix it in workingcontext.flags()/manifest.
% add file
adding qqq.txt
% commit first revision
% set bookmark
% commit second revision
% set bookmark
% update to -2
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% commit new head
created new head
% bookmarks updated?
test 1:16b24da7e457
test2 1:16b24da7e457
% strip to revision 1
saving bundle to
saving bundle to
adding branch
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
% list bookmarks
* test 1:9f1b7e78eff8
* test2 1:9f1b7e78eff8