API should list commits

[lunny]

This is a feature request for the API to support more details for reporting. I would like to access:

• List commits (just like GitHub)

This would enable reporting integration to query the api to determine, over a period:

• number of commits in a repo

splitted from #760

[HoffmannP]

I'm currently working on it (only the GET-Part b/c see following:)
The CommitAPI depends on the TreeAPI so I woun't be able to fully implement it until #4185 is done.
POSTing a commit will be absolutely postponed until the TreeAPI is done as it doesn't make sense to create an commit without a tree object.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 weeks. Thank you for your contributions.

[stale]

This issue has been automatically closed because of inactivity. You can re-open it if needed.

[richmahn]

@HoffmannP #4185 has been merged and closed, so it is done.

[sapk]

The github api style is done in #6408.