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.
Let the ".ninja parse" metric include the time to read the toplevel ninja file.

Loads of included ninja files were covered by the ".ninja parse" in Parse()
further up the stack from the toplevel file, but the load of the toplevel
file wasn't counted. Fix that.
Add back contents.resize(), but with a comment and just 1 instead of 10.
Merge pull request #621 from nico/ninjaload

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