adjust unlocked execute bit handling

adjust unlocked execute bit handling

When building an adjusted unlocked branch, make pointer files executable
when the annex object file is executable.

This slows down git-annex adjust --unlock/--unlock-present by needing to
stat all annex object files in the tree. Probably not a significant
slowdown compared to other work they do, but I have not benchmarked.

I chose to leave git-annex adjust --unlock marked as stable, even though
get or drop of an object file can change whether it would make the pointer
file executable. Partly because making it unstable would slow down
re-adjustment, and partly for symmetry with the handling of an unlocked
pointer file that is executable when the content is dropped, which does not
remove its execute bit.