fix setting of lines-changed-only (#90)

The README states that the value of true focuses on
lines in diff with additions only.
Setting it to 'diff' will use the original behavior (meaning
entire diff chunks with unchanged lines).

However, I forgot to change this value's
interpretation in the src code (& test).

This commit makes the README accurate.

cpp_linter/run.py

@@ -122,7 +122,7 @@
     "-l",
     "--lines-changed-only",
     default=0,
-    type=lambda a: 1 if a.lower() == "true" else (2 if a.lower() == "strict" else 0),
+    type=lambda a: 2 if a.lower() == "true" else (1 if a.lower() == "diff" else 0),
     help="Set this option to 'true' to only analyze changes in the event's diff. "
     "Set this to 'strict' to only analyze additions in the event's diff. "
     "Defaults to %(default)s.",

tests/test_cli_args.py

@@ -57,8 +57,8 @@ def test_defaults():
         ("extensions", "cxx,.hpp", "extensions", ["cxx", "hpp"]),
         ("repo-root", "src", "repo_root", "src"),
         ("ignore", "!src|", "ignore", "!src|"),
-        ("lines-changed-only", "True", "lines_changed_only", 1),
-        ("lines-changed-only", "stricT", "lines_changed_only", 2),
+        ("lines-changed-only", "True", "lines_changed_only", 2),
+        ("lines-changed-only", "Diff", "lines_changed_only", 1),
         ("files-changed-only", "True", "files_changed_only", True),
         ("thread-comments", "True", "thread_comments", True),
         ("file-annotations", "False", "file_annotations", False),