Support filtering commits by datetime

[joeng03]

Refer to #2388

We currently support filtering commits by date only.

Since we currently group the commits by dates, and each date has multiple commits, we can't do filtering based on datetime as of now. Propose to change the structure of authorDailyContributionsMap to support this.

To minimize changes to the codebase, I suggest that we add an additional field for datetime to each commit grouped under the same date. When filtering, we only have to extend the existing filtering logic to filter the datetime for the first and last filtered date.

For example, if "since" is 2020-01-01T00:02 and "until" is 2020-01-15T23:00, then we only have to filter the dates as usual, then do additional filtering for commits grouped under 2020-01-01 for time >=00:02, and commits grouped under 2020-01-15 for time <=23:00.