@kitbellew the Exception that is throw is the one below:

LibGit2Sharp.EmptyCommitException: No changes; nothing to commit.

Which is raised when the commit being created bears no difference with its parent. I think this issue actually share the same root cause than #688.

Can you please confirm that the issue disappears if you (hackily) introduce a Thread.Sleep() between the two calls to WriteAndCommit?

@nulltoken Emeric, thank you very much, adding a Thread.Sleep(1000) or storing "ab" in the second invocation (thus changing the file size) both fix the problem.

Should I close this? (After #688 is fixed, a test would be nice.)

@kitbellew Glad I could help. Sorry for the less than awesome workaround.

Should I close this? (After #688 is fixed, a test would be nice.)

Let's close this one. #688 won't be closed until the test code in the issue description is committed into StageFixture.cs and passes.

@kitbellew I turned issue #688 into a PR with the said failing test