contrib/gcc-changelog: Check whether revert-commit exists

contrib/ChangeLog:

	* gcc-changelog/git_commit.py (GitCommit.__init__):
	Handle commit_to_info_hook = None; otherwise, if None,
	regard it as error.
	(to_changelog_entries): Handle commit_to_info_hook = None;
	if info is None, create a warning for it.
	* gcc-changelog/git_email.py (GitEmail.__init__):
	call super() with commit_to_info_hook=None instead
	of a lamda function.

 contrib/gcc-changelog/git_commit.py | 14 +++++++++-----
 contrib/gcc-changelog/git_email.py  |  3 +--
 2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/contrib/gcc-changelog/git_commit.py b/contrib/gcc-changelog/git_commit.py
index 4f3131021f2..4d024026f2b 100755
--- a/contrib/gcc-changelog/git_commit.py
+++ b/contrib/gcc-changelog/git_commit.py
@@ -329,11 +329,13 @@ class GitCommit:
                 self.revert_commit = m.group('hash')
                 break
         if self.revert_commit:
+            # The following happens for get_email.py:
+            if not self.commit_to_info_hook:
+                return
             self.info = self.commit_to_info_hook(self.revert_commit)
-
-        # The following happens for get_email.py:
-        if not self.info:
-            return
+            if not self.info:
+                self.errors.append(Error('Cannot find to-be-reverted commit', self.revert_commit))
+                return
 
         self.check_commit_email()
 
@@ -796,12 +798,14 @@ class GitCommit:
                 orig_date = self.original_info.date
                 current_timestamp = orig_date.strftime(DATE_FORMAT)
             elif self.cherry_pick_commit:
-                info = self.commit_to_info_hook(self.cherry_pick_commit)
+                info = (self.commit_to_info_hook
+                        and self.commit_to_info_hook(self.cherry_pick_commit))
                 # it can happen that it is a cherry-pick for a different
                 # repository
                 if info:
                     timestamp = info.date.strftime(DATE_FORMAT)
                 else:
+                    self.warnings.append(f"Cherry-picked commit not found: '{self.cherry_pick_commit}'")
                     timestamp = current_timestamp
             elif not timestamp or use_commit_ts:
                 timestamp = current_timestamp
diff --git a/contrib/gcc-changelog/git_email.py b/contrib/gcc-changelog/git_email.py
index 49f41f2ec99..93808dfabb6 100755
--- a/contrib/gcc-changelog/git_email.py
+++ b/contrib/gcc-changelog/git_email.py
@@ -89,8 +89,7 @@ class GitEmail(GitCommit):
                 t = 'M'
             modified_files.append((target if t != 'D' else source, t))
         git_info = GitInfo(None, date, author, message, modified_files)
-        super().__init__(git_info,
-                         commit_to_info_hook=lambda x: None)
+        super().__init__(git_info, commit_to_info_hook=None)
 
 
 def show_help():