fix: default to GITHUB_SHA if the commit received from context is emp…

…ty (#30)

When action is triggered in non-pull-request events, the default being set would be empty. In such cases, CLI would try to fetch head commit by executing git commands.
That's going to fail for this action if safe directory isn't added.

main.py

@@ -28,6 +28,20 @@ def main() -> None:
     """
 
     input_data = {key: os.getenv(value) for key, value in INPUT_KEYS_MAP.items()}
+    commit_sha = input_data["commit_sha"]
+
+    # Case: When user hasn't set the commit sha
+    # or, when this action is run on a non-pull request event, the default value is going to
+    # be an empty string.
+    # in such case, fetch commit sha from the `GITHUB_SHA` environment variable.
+    # Doing this is the best way to go because of the following reasons:
+    # 1. we don't have to run any git commands to fetch the commit sha,
+    # making the action work all the time
+    # 2. the default being set to PR's head sha will take care of merge commit.
+    # In all other cases, GITHUB_SHA would be accurate.
+    # 3. GITHUB_SHA is always set, so we don't have to worry about it being empty.
+    if not commit_sha:
+        commit_sha = os.getenv("GITHUB_SHA")
 
     command = [
         DEEPSOURCE_CLI_PATH,
@@ -49,7 +63,7 @@ def main() -> None:
         env=dict(
             os.environ,
             DEEPSOURCE_DSN=input_data["dsn"],
-            GHA_HEAD_COMMIT_SHA=input_data["commit_sha"],
+            GHA_HEAD_COMMIT_SHA=commit_sha,
         ),
         capture_output=True,
     )