Fix to use root gitignore in every source entry

When passing multiple src directories, the root gitignore was only
applied to the first processed source. The reason is that, in the
first source, exclude is `None`, but then the value gets overridden by
`re_compile_maybe_verbose(DEFAULT_EXCLUDES)`, so in the next iteration
where the source is a directory, the condition is not met and sets the
value of `gitignore` to `None`.

To fix this problem, we store a boolean indicating if `exclude` is
`None` and set the value of `exclude` to its default value if that's
the case. This makes sure that the flow enters the correct condition on
following iterations and also keeps the original value if the condition
is not met.

Also, the value of `gitignore` is initialized as `None` and overriden
if necessary. The value of `root_gitignore` is always calculated to
avoid using additional variables (at the small cost of additional
computations).

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

src/black/__init__.py

@@ -30,6 +30,7 @@
 import click
 from click.core import ParameterSource
 from mypy_extensions import mypyc_attr
+from pathspec import PathSpec
 from pathspec.patterns.gitwildmatch import GitWildMatchPatternError
 
 from _black_version import version as __version__
@@ -625,6 +626,11 @@ def get_sources(
     sources: Set[Path] = set()
     root = ctx.obj["root"]
 
+    exclude_is_None = exclude is None
+    exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES) if exclude is None else exclude
+    gitignore = None  # type: Optional[PathSpec]
+    root_gitignore = get_gitignore(root)
+
     for s in src:
         if s == "-" and stdin_filename:
             p = Path(stdin_filename)
@@ -658,16 +664,14 @@ def get_sources(
 
             sources.add(p)
         elif p.is_dir():
-            if exclude is None:
-                exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES)
-                gitignore = get_gitignore(root)
+            if exclude_is_None:
                 p_gitignore = get_gitignore(p)
                 # No need to use p's gitignore if it is identical to root's gitignore
                 # (i.e. root and p point to the same directory).
-                if gitignore != p_gitignore:
-                    gitignore += p_gitignore
-            else:
-                gitignore = None
+                if root_gitignore == p_gitignore:
+                    gitignore = root_gitignore
+                else:
+                    gitignore = root_gitignore + p_gitignore
             sources.update(
                 gen_python_files(
                     p.iterdir(),