Option to use file characteristics instead of timestamps

As noted in other issues, using timestamps to determine whether to rebuild something can be problematic. While timestamps are convenient and relatively fast, it is often desirable to key build decisions on some characteristic intrinsic to the file itself, such as a hash of its contents.

I use git a lot and it is annoying that simply changing branches triggers rebuilds. Ideally, I would be able to switch from my current work branch to some other branch, not touch any files and then switch back to the original branch and not have to rebuild anything at all. As far as I know that is not possible if the build system uses timestamps. Using file hashes would solve this particular issue.

I understand the using file hashes or other such intrisic file characteristics could slow down Ninja so using them should be an option.