GitHub API – how to compare 2 commits

The official commit comparison API is Compare two commits:

GET /repos/:owner/:repo/compare/:base...:head

Both :base and :head can be either branch names in :repo or branch names in other repositories in the same network as :repo. For the latter case, use the format user:branch:

GET /repos/:owner/:repo/compare/user1:branchname...user2:branchname

Note that you can use tags or commit SHAs as well.
For instance:

https://api.github.com/repos/git/git/compare/v2.2.0-rc1…v2.2.0-rc2

Note the ‘...‘, not ‘..‘ between the two tags.
And you need to have the oldest tag first, then the newer tag.

That gives a status:

  "status": "behind",
  "ahead_by": 1,
  "behind_by": 2,
  "total_commits": 1,

And for each commit, information about the files:

"files": [
    {
      "sha": "bbcd538c8e72b8c175046e27cc8f907076331401",
      "filename": "file1.txt",
      "status": "added",
      "additions": 103,
      "deletions": 21,
      "changes": 124,
      "blob_url": "https://github.com/octocat/Hello-World/blob/6dcb09b5b57875f334f61aebed695e2e4193db5e/file1.txt",
      "raw_url": "https://github.com/octocat/Hello-World/raw/6dcb09b5b57875f334f61aebed695e2e4193db5e/file1.txt",
      "contents_url": "https://api.github.com/repos/octocat/Hello-World/contents/file1.txt?ref=6dcb09b5b57875f334f61aebed695e2e4193db5e",
      "patch": "@@ -132,7 +132,7 @@ module Test @@ -1000,7 +1000,7 @@ module Test"
    }
  ]

BUT:

• The response will include a comparison of up to 250 commits. If you are working with a larger commit range, you can use the Commit List API to enumerate all commits in the range.

• For comparisons with extremely large diffs, you may receive an error response indicating that the diff took too long to generate. You can typically resolve this error by using a smaller commit range.

Notes:

“same network” means: two repositories hosted by the same Git repository hosting services (two repositories on github.com for example, or on the same on-premise GHE — GitHub Enterprise — instance)

You can therefore compare two branches between a repo and its fork.
Example:

https://api.github.com/repos/030/learn-go-with-tests/compare/master...quii:master

• compare link
• diff link

(this example compares a fork to its original repo, not the original repo to the fork: that is because the fork, in this case, is behind the original repo)

As noted by Tom Carver in the comments:

this suggested API silently maxes out at 300 files shown;
I haven’t yet found an API that avoids this limitation