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
# Source bundle was generated with the following script.
#
# hg init
# echo a > a
# ln -s a l
# hg ci -Ama -d'0 0'
# mkdir b
# echo a > b/a
# chmod +x b/a
# hg ci -Amb -d'1 0'
hg init
hg -q pull "$TESTDIR/test-manifest.hg"
echo % should be empty
hg manifest
hg co
hg manifest
hg manifest -v
hg manifest --debug
hg manifest -r 0
hg manifest -r 1
hg manifest -r tip
echo % should fail
hg manifest -r 2
hg manifest -r tip tip
hg manifest tip