git diff SHA1^ SHA1 /path/to/file.js

$ git show d8b396e /t/t7509-commit.sh
fatal: ambiguous argument '/t/t7509-commit.sh': unknown
revision or path not in the working tree.

$ git show d8b396e t/t7509-commit.sh
commit d8b396e17ecfe28b39b5f4470f791c434cce40ec
Author: Fabian Ruch ...
...
diff --git a/t/t7509-commit.sh b/t/t7509-commit.sh
index b61fd3c..9ac7940 100755
--- a/t/t7509-commit.sh
+++ b/t/t7509-commit.sh