Add new git-backport.py script.

contrib/ChangeLog:

	* git-backport.py: New file.

contrib/git-backport.py

+#!/usr/bin/env python3
+
+# Copyright (C) 2020 Free Software Foundation, Inc.
+#
+# This file is part of GCC.
+#
+# GCC 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, or (at your option)
+# any later version.
+#
+# GCC 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 GCC; see the file COPYING.  If not, write to
+# the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+# Boston, MA 02110-1301, USA.
+
+import argparse
+import subprocess
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser(description='Backport a git revision and '
+                                     'stash all ChangeLog files.')
+    parser.add_argument('revision', help='Revision')
+    args = parser.parse_args()
+
+    r = subprocess.run('git cherry-pick -x %s' % args.revision, shell=True)
+    if r.returncode == 0:
+        cmd = 'git show --name-only --pretty="" -- "*ChangeLog" |' \
+              'xargs git checkout HEAD~'
+        subprocess.check_output(cmd, shell=True)
+        subprocess.check_output('git commit --amend --no-edit', shell=True)
+    else:
+        # 1) remove all ChangeLog files from conflicts
+        out = subprocess.check_output('git diff --name-only --diff-filter=U',
+                                      shell=True,
+                                      encoding='utf8')
+        conflicts = out.strip().split('\n')
+        changelogs = [c for c in conflicts if c.endswith('ChangeLog')]
+        if changelogs:
+            cmd = 'git checkout --theirs %s' % '\n'.join(changelogs)
+            subprocess.check_output(cmd, shell=True)
+        # 2) remove all ChangeLog files from index
+        cmd = 'git diff --name-only --diff-filter=M HEAD'
+        out = subprocess.check_output(cmd, shell=True, encoding='utf8')
+        out = out.strip().split('\n')
+        modified = [c for c in out if c.endswith('ChangeLog')]
+        for m in modified:
+            subprocess.check_output('git reset %s' % m, shell=True)
+            subprocess.check_output('git checkout %s' % m, shell=True)
+
+        # try to continue
+        if len(conflicts) == len(changelogs):
+            subprocess.check_output('git cherry-pick --continue', shell=True)
+        else:
+            print('Please resolve all remaining file conflicts.')