scripts/download.py: use a more terse api for fetching git commit date

scripts/download.py: use a more terse api for fetching git commit date

The previous api [1] includes in its response patch data among other
things, as such the response size can vary and be big.  Use another
api[2] to improve it a bit

 [1] Get a single commit, Repositories, https://developer.github.com/v3/repos/commits/#get-a-single-commit
 [2] Git Commits, Git Data, https://developer.github.com/v3/git/commits/#get-a-commit

Signed-off-by: Yousong Zhou <[email protected]>