honor exclude when specifying file path

fixes #438

I Have not added or amended tests yet, but if we agree on the approach, I can work on them

black.py

@@ -441,8 +441,10 @@ def main(
                 gen_python_files_in_dir(p, root, include_regex, exclude_regex, report)
             )
         elif p.is_file() or s == "-":
-            # if a file was explicitly given, we don't care about its extension
-            sources.add(p)
+            # if a file was explicitly given, we bypass include
+            sources.update(
+                gen_python_files([p], root, include=None, exclude=exclude_regex, report=report)
+            )
         else:
             err(f"invalid path: {s}")
     if len(sources) == 0:
@@ -3406,10 +3408,10 @@ def get_imports_from_children(children: List[LN]) -> Generator[str, None, None]:
     return imports
 
 
-def gen_python_files_in_dir(
-    path: Path,
+def gen_python_files(
+    paths: [Path],
     root: Path,
-    include: Pattern[str],
+    include: Optional[Pattern[str]],
     exclude: Pattern[str],
     report: "Report",
 ) -> Iterator[Path]:
@@ -3421,7 +3423,7 @@ def gen_python_files_in_dir(
     `report` is where output about exclusions goes.
     """
     assert root.is_absolute(), f"INTERNAL ERROR: `root` must be absolute but is {root}"
-    for child in path.iterdir():
+    for child in paths:
         try:
             normalized_path = "/" + child.resolve().relative_to(root).as_posix()
         except ValueError:
@@ -3444,11 +3446,22 @@ def gen_python_files_in_dir(
             yield from gen_python_files_in_dir(child, root, include, exclude, report)
 
         elif child.is_file():
-            include_match = include.search(normalized_path)
-            if include_match:
+            # if an include regex is not provided, the file is included
+            if not include or include.search(normalized_path):
                 yield child
 
 
+def gen_python_files_in_dir(
+    path: Path,
+    root: Path,
+    include: Pattern[str],
+    exclude: Pattern[str],
+    report: "Report",
+) -> Iterator[Path]:
+    return gen_python_files(path.iterdir())
+
+
+
 @lru_cache()
 def find_project_root(srcs: Iterable[str]) -> Path:
     """Return a directory containing .git, .hg, or pyproject.toml.