Try harder if `am` --3way fails

[rconradharris]

Right now we just give up ungracefully when am --3way fails.

There might be ways to try harder on behalf of the user.

For example, git apply --3way seems to occasionally work even if an am fails (since it doesn't call build_fake_ancestors).

Placeholder for considering this.

A better solution would be to fix the underlying missing blob issue if we can figure out exactly what's going on here.