Reuse ManifestParser::Load() in ManifestParser::ParseFileInclude().

ParseFileInclude() was doing the work that Load() was doing. Instead, just
make it call Load().

Also, remove a FIXME about using ReadPath() in ParseFileInclude() -- it's
already being used. (The FIXME was added in the same commit that added the
call to ReadPath() -- 8a0c96075786c19 -- it probably should've been deleted
before that commit.)

Also, remove a `contents.resize(contents.size() + 10);`. It's not clear what
it's for, but if it was important then ManifestParser::ParseFileInclude()
would have needed that call too, and it didn't have it.

No intended behavior change.