Changes view performance problems

[StanleyGoldman]

As described in github-beta/unity-preview#29, the Changes view can't handle more than 1500 files (probably less on less performant machines), so we have some improvements to make on this area.

[shiena]

Commit details view also has perfomance problems.
For example, this commit has changed 313 files. github-for-unity will be slower if you open it in the commit detail view.
unity3d-jp/piranhan@7bbfaa9

[meaghanlewis]

@StanleyGoldman this is an issue we can close out when we release the latest version, right?

[StanleyGoldman]

[StanleyGoldman]