[NinjaCommand] Change load-manifest to chdir() to containing directory for input.
[NinjaCommand] Change load-manifest to chdir() to containing directory for input.

 - This is just a hack for now so that include statements can use relative paths
   (sadly Ninja does not seem to specify that include statements are relative to
   the including file, which is terribly broken).


apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild