[gdb/contrib] Check clean files in check-whitespace-pre-commit.sh

The pre-commit check check-whitespace checks diffs.

Consequently, we detect something like this:
...
$ echo >> gdb/testsuite/lib/gdb.exp
$ git commit -a -m trailing-empty-line
  ...
check-whitespace.........................................................Failed
- hook id: check-whitespace
- duration: 0.01s
- exit code: 2

gdb/testsuite/lib/gdb.exp:11717: new blank line at EOF.
...

But say we work around the failing check using --no-verify, then we no longer
detect it after the commit has succeeded:
...
$ git commit -a -m trailing-empty-line --no-verify
[detached HEAD e6302105522] trailing-empty-line
 1 file changed, 1 insertion(+)
$ pre-commit run --all-files check-whitespace
check-whitespace.........................................................Passed
- hook id: check-whitespace
- duration: 0.3s
...

Fix this in check-whitespace-pre-commit.sh by distinguishing between clean and
other files.  Doing so is easier to do in a more advanced scripting language,
so rewrite into python.

Since a recent commit, gdb/testsuite is clean, so I'm using that as
simple classifier for now.

For the other files we do what we did before, and check just the staging area:
...
$ git --no-pager diff --staged --check "${other[@]}"
...

But for clean files, we check the entire file, including staged changes:
...
$ empty=$(git hash-object -t tree /dev/null)
$ git diff-index --cached --check $empty "${clean[@]}"
...

Consequently, we do see:
...
$ git commit -a -m trailing-empty-line --no-verify
[detached HEAD e6302105522] trailing-empty-line
 1 file changed, 1 insertion(+)
$ pre-commit run --all-files check-whitespace
check-whitespace.........................................................Failed
- hook id: check-whitespace
- duration: 0.64s
- exit code: 2

gdb/testsuite/lib/gdb.exp:11717: new blank line at EOF.
...

PR build/33616
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33616

1 files changed, 52 insertions, 0 deletions

diff --git a/gdb/contrib/check-whitespace-pre-commit.py b/gdb/contrib/check-whitespace-pre-commit.py
new file mode 100755
index 0000000..8326280
--- /dev/null
+++ b/gdb/contrib/check-whitespace-pre-commit.py
@@ -0,0 +1,52 @@
+#! /usr/bin/env python3
+
+# Copyright (C) 2025 Free Software Foundation, Inc.
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import re
+import subprocess
+import sys
+
+re_clean = re.compile("(^|/)gdb/testsuite/")
+re_ignore = re.compile("/configure$")
+
+clean = []
+other = []
+for f in sys.argv[1:]:
+    m = re_ignore.search(f)
+    if m:
+        continue
+
+    m = re_clean.search(f)
+    if m:
+        clean.append(f)
+    else:
+        other.append(f)
+
+if other:
+    cmd = ["git", "--no-pager", "diff", "--staged", "--check"] + other
+    res = subprocess.run(cmd)
+    if res.returncode != 0:
+        sys.exit(res.returncode)
+
+if clean:
+    cmd = ["git", "hash-object", "-t", "tree", "/dev/null"]
+    res = subprocess.run(cmd, capture_output=True, text=True)
+    if res.returncode != 0:
+        sys.exit(res.returncode)
+    null_tree = res.stdout.rstrip("\n")
+
+    cmd = ["git", "diff-index", "--cached", "--check", null_tree] + clean
+    res = subprocess.run(cmd)
+    sys.exit(res.returncode)