Support merging branches with a large number of commits

[le-git-imate]

Description of issue or feature request
One of the very first step in a merge commit is to compare the PR and master branch to determine the changes (i.e., added/deleted/modified files). le-git-imate uses different APIs to make the comparison on GitHub and GitLab.
However, those API may fail to function correctly if the number of commits in the PR exceeds a limit. On GitHub you can use the CompareBranch API to compare up to 250 commits. And if the number of changed files in a PR goes over 300, we can't simply use one API to list the files.
On GitLab, a diff API between two branches/commits may completely collapse if:

• 50,000 lines have already been rendered.
• 1000 files have already been rendered.

This issue could be address with #11.

Current behavior

• le-git-imate does not support merging large PRs

Expected behavior

• Support large merge commits which needs to
  -- first know all limitations we have when we use GitHub API to compare branches.
  -- second use alternative solutions like what is suggested by GitHub.