Make git unit test not flaky

[bluwy]

I saw in CI this part

ghcommit/tests/git.test.ts

Lines 49 to 62 in 506855a

	additions: [
	{
	path: "a.txt",
	contents: await fixture.readFile("a.txt", "base64"),
	},
	{
	path: "c.txt",
	contents: await fixture.readFile("c.txt", "base64"),
	},
	{
	path: "nested/file.txt",
	contents: await fixture.readFile("nested/file.txt", "base64"),
	},
	],

could fail because the order may be non-deterministic. (c.txt before a.txt)

Added a sort to fix this. Shouldn't affect people in practice but I think it's good to sort in the implementation side so that the requests we send are also deterministic.