Refactor PackObject, LooseObject into GitObject.

Combine different objectTypes into GitObject.
Rename PackObject => PackedObject

Fix nits and indents.
nits.
Using ace to calculate row rather than using markers' (so wrapping works)
fixed nit.
Merge pull request #888 from dart-lang/fix_push

Refactor PackObject, LooseObject
Merge branch 'master' of https://github.com/dart-lang/spark into umop-annotations-find_column