feat(commits): list commits since a commitish

[sidvishnoi]

For https://github.com/w3c/respec/issues/1724

The commits are listed in same order as regular git log.

How the caching works? Without the cache, we get the since (date) for given commitish.
After fetching once, we set the since to the date of current HEAD. With cache, we start the search from the new since date, instead of from ref's.

Caching benefits (fetching last 504 commits from a repository):

➜  node commits.js
Failed to load cache: gh/commits.
# timestamp   # of records fetched
1570788384931 100
1570788386215 100
1570788387373 100
1570788388495 100
1570788389544 100
1570788390002 4

➜  node commits.js
1570788394641 504
1570788395097 0

[marcoscaceres]

Lgtm!