Fix listing of modified files

Consider the merge-base commit as the starting point of diff instead of listing all differences between branches.

megalinter/MegaLinter.py

@@ -773,10 +773,10 @@ def list_files_git_diff(self):
                 "HEAD" if default_branch == "HEAD" else f"refs/heads/{default_branch}"
             )
             local_ref = f"refs/remotes/{default_branch_remote}"
-            # Try to fetch default_branch from origin, because it'sn't cached locally.
+            # Try to fetch default_branch from origin, because it isn't cached locally.
             repo.git.fetch("origin", f"{remote_ref}:{local_ref}")
         # Make git diff to list files (and exclude symlinks)
-        diff = repo.git.diff(default_branch_remote, name_only=True)
+        diff = repo.git.diff(f"{default_branch_remote}...", name_only=True)
         logging.info(f"Modified files:\n{diff}")
         all_files = list()
         for diff_line in diff.splitlines():