GitHub Importer - Import commit comments
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## Description
On GitHub, much like on GitLab, a user can comment on a commit message in the repo. It appears that these commit comments do not get transferred when importing a GitHub repo into GitLab. It would be great if these comments could be included.
## Proposal
As there are not likely to be many comments on arbitrary commits as most discussion likely occurs in PRs, it is probably easiest to just get a list of all comments on commits from GitHub's API and then apply them one by one to the GitLab repo.
## Links
* GitHub API ref: https://developer.github.com/v3/repos/comments/#list-commit-comments-for-a-repository
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