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.