Provide commit sha to start history with

[masim05]

Closes #206

Current high level logic of gren.js release --data-source=commits is:

• determine a date range
• pick commits in master branch within this range
• create release notes using the commits

As a result, the following workflow is problematic (release notes are empty):

• create a new release branch from master
• merge to the release branch new changes
• tag the release branch
• generate release notes from the tag
• merge release to master

One option to fix it is to provide sha to start history with. Currently it is not provided so default master is always used.

[codecov-io]

Codecov Report

Merging #207 into master will decrease coverage by 45.84%. The diff coverage is 0%.

@@             Coverage Diff             @@
##           master     #207       +/-   ##
===========================================
- Coverage   84.24%   38.39%   -45.85%     
===========================================
  Files           8        8               
  Lines         349      349               
===========================================
- Hits          294      134      -160     
- Misses         55      215      +160

Impacted Files	Coverage Δ
lib/src/Gren.js	5.26% <0%> (-76.08%)	:arrow_down:
lib/src/GitHubInfo.js	80.95% <0%> (-4.77%)	:arrow_down:

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