Add fetch-commit command

[jwodder]

Closes #162.

[yarikoptic]

not sure why there is no codecov report, but from https://app.codecov.io/gh/con/tinuous/pull/171 HEAD 65.40% Patch 42.79% Change 2.62%. Since coverage of the patch is relatively low I wonder if you had a chance to try this PR on some sample repos to ensure that there is no regressions?

[jwodder]

@yarikoptic I successfully ran fetch-commit on datalad's maint branch, but I did not test regular fetch.

[yarikoptic]

@yarikoptic I successfully ran fetch-commit on datalad's maint branch, but I did not test regular fetch.

Please check on some of the existing setups (e.g. for datalad) while going a commit or two back and then doing diff on either we got the same result.

[jwodder]

@yarikoptic I used tinuous — both this branch and master — to fetch all logs for datalad for the 15th, and the results were the same.