New perf dashboard UI tries to fetch commits all the time

New perf dashboard UI tries to fetch commits all the time
https://bugs.webkit.org/show_bug.cgi?id=137592

Reviewed by Andreas Kling.

Added hasReportedCommits boolean to repository meta data in manifest.json, and used that in
the front end to avoid issuing HTTP requests to fetch commit logs for repositories with
no reported commits as they are all going to fail.

Also added an internal cache to FetchCommitsForTimeRange in the front end to avoid fetching
the same commit logs repeatedly. There are two data structures we cache: commitsByRevision
which maps a given commit revision/hash to a commit object; and commitsByTime which is an array
of commits sorted chronologically by time.

* public/include/manifest.php:

* public/v2/app.js:
(App.CommitsViewerComponent.commitsChanged):

* public/v2/data.js:
(FetchCommitsForTimeRange):
(FetchCommitsForTimeRange._cachedCommitsByRepository):

* public/v2/manifest.js:
(App.Repository):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@174555 268f45cc-cd09-0410-ab3c-d52691b4dbfc