Extend the until/since parameters for merge commit hashes

[jvcoutinho]

Problem
Some analysis take too much time to complete, so then its frequent to stop the analysis and continue later. Unfortunately, doing that requires some pain.

Cause
We can use the current since and until parameters if we know the date of the last analysed commit. Unfortunately, this requires to go to GitHub and search for it (or to know some magical tricks with git's commands). This could be automatized.

Suggestion
Allow the user to input a hash of a merge commit in the since or until parameters. The MF will end the analysis from that merge commit in the case of since, and it will start the analysis with it in the case of until.

It also helps developers when debugging a single commit or a subset of commits.