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 set the new value of `exclude` is a new
variable (`_exclude`). This prevents entering the wrong condition on
the second iteration and also keeps the origina value if the condition
is not met.

Also, the value of both `_exclude` and `gitignore` is calculated only
once by using `None` as default value. This help us to reduce the
number of computations (only calculate the value once, if needed), and
also retains the original value of `gitignore`, which is then merged
with the value of `p_gitignore`. That's why we also define
`_gitignore` to contain the result of the merge (add) operation.

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

src/black/__init__.py

@@ -25,6 +25,7 @@
     Sized,
     Tuple,
     Union,
+    cast,
 )
 
 import click
@@ -625,6 +626,9 @@ def get_sources(
     sources: Set[Path] = set()
     root = ctx.obj["root"]
 
+    _exclude = exclude
+    _gitignore = gitignore = None
+
     for s in src:
         if s == "-" and stdin_filename:
             p = Path(stdin_filename)
@@ -659,25 +663,27 @@ 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:
+                    _exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES)
+                if gitignore is None:
+                    gitignore = get_gitignore(root)
                 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 gitignore == p_gitignore:
+                    _gitignore = gitignore
+                else:
+                    _gitignore = gitignore + p_gitignore
             sources.update(
                 gen_python_files(
                     p.iterdir(),
                     ctx.obj["root"],
                     include,
-                    exclude,
+                    cast(Pattern[str], _exclude),
                     extend_exclude,
                     force_exclude,
                     report,
-                    gitignore,
+                    _gitignore,
                     verbose=verbose,
                     quiet=quiet,
                 )