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
# 0-2-4
# \ \ \
# 1-3-5
#
# rename in #1, content change in #4.
hg init t
cd t
touch 1
touch 2
hg commit -Am init # 0
hg rename 1 1a
hg commit -m rename # 1
hg co -C 0
echo unrelated >> 2
hg ci -m unrelated1 # 2
hg merge --debug 1
hg ci -m merge1 # 3
hg co -C 2
echo hello >> 1
hg ci -m unrelated2 # 4
hg co -C 3
hg merge -y --debug 4
hg co -C 4
hg merge -y --debug 3