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.
#!/bin/sh
"$TESTDIR/hghave" inotify || exit 80
hg init
echo "[extensions]" > .hg/hgrc
echo "inotify=" >> .hg/hgrc
hg inserve -d --pid-file .hg/inotify.pid
echo a > a
hg ci -Aqm0
hg co -q null
hg co -q
hg st
cat a
kill `cat .hg/inotify.pid`