hooks: check_black: pass stdin argument and check returncode

When checking file content from git via stdin, black requires '-'
as an argument to read from standard input. In addition, black signals
formatting issues or errors through its exit code (0 if formatted,
non-zero if formatting is needed) rather than stdout.

Update check_black to:
- Pass '-q' and '-' to black to quietly check stdin.
- Check result.returncode instead of result.stdout.
- Provide a default error message when stdout is empty.

Bug: 488338724
Test: python3 -m pytest rh/hooks_unittest.py
Change-Id: Iee44d1153c3d793d65e81883e8418126a798ac39
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repohooks/+/647263
Tested-by: Josh Wu <joshwu@google.com>
Commit-Queue: Josh Wu <joshwu@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

rh/hooks.py

diff --git a/rh/hooks.py b/rh/hooks.py
index 9627ae8..bad744e 100644
--- a/rh/hooks.py
+++ b/rh/hooks.py
@@ -440,20 +440,20 @@
 
     tool = options.tool_path("black")
     tool_options = options.args((), filtered)
-    cmd = [tool, "--check"] + tool_options
+    cmd = [tool, "-q", "--check", "-"] + tool_options
     fixup_cmd = [tool] + tool_options + ["--"]
 
     ret = []
     for d in filtered:
         data = rh.git.get_file_content(commit, d.file)
         result = _run(cmd, input=data)
-        if result.stdout:
+        if result.returncode:
             ret.append(
                 rh.results.HookResult(
                     "black",
                     project,
                     commit,
-                    error=result.stdout,
+                    error=result.stdout or "Files not formatted",
                     files=(d.file,),
                     fixup_cmd=fixup_cmd,
                 )