git diff输出中a /和b /对应于哪些提交？

Question

我正面临与本教程类似的示例： https://www.atlassian.com/git/tutorials/saving-changes/git-diff

执行git diff：

git diff commit1 commit2

我得到以下输出：

diff --git a/diff_test.txt b/diff_test.txt
index 6b0c6cf..b37e70a 100644
--- a/diff_test.txt
+++ b/diff_test.txt
@@ -1 +1 @@
-this is a git diff test example
+this is a diff example

我怎么知道哪个提交是a，哪个提交是b？