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
# basic test for hg debugrebuildstate
hg init repo
cd repo
touch foo bar
hg ci -Am 'add foo bar'
touch baz
hg add baz
hg rm bar
hg debugrebuildstate
echo '% state dump after'
hg debugstate --nodates | sort
echo '% status'
hg st -A