Add -v option for git_check_commit.py.

Doing so, one can see:
$ git gcc-verify a50914d2111c72d2cd5cb8cf474133f4f85a25f6 -v
Checking a50914d2111c72d2cd5cb8cf474133f4f85a25f6: FAILED
ERR: unchanged file mentioned in a ChangeLog: "gcc/common.opt"
ERR: unchanged file mentioned in a ChangeLog (did you mean "gcc/testsuite/g++.dg/pr102955.C"?): "gcc/testsuite/gcc.dg/pr102955.c"
- gcc/testsuite/gcc.dg/pr102955.c
?                ^^             ^

+ gcc/testsuite/g++.dg/pr102955.C
?                ^^             ^

contrib/ChangeLog:

	* gcc-changelog/git_check_commit.py: Add -v option.
	* gcc-changelog/git_commit.py: Print verbose diff for wrong
	filename.

contrib/gcc-changelog/git_check_commit.py

@@ -29,6 +29,8 @@ parser.add_argument('-g', '--git-path', default='.',
                     help='Path to git repository')
 parser.add_argument('-p', '--print-changelog', action='store_true',
                     help='Print final changelog entires')
+parser.add_argument('-v', '--verbose', action='store_true',
+                    help='Print verbose information')
 args = parser.parse_args()
 
 retval = 0
@@ -41,6 +43,8 @@ for git_commit in parse_git_revisions(args.git_path, args.revisions):
     else:
         for error in git_commit.errors:
             print('ERR: %s' % error)
+            if args.verbose and error.details:
+                print(error.details)
         retval = 1
 
 exit(retval)

contrib/gcc-changelog/git_commit.py

@@ -197,9 +197,10 @@ def decode_path(path):
 
 
 class Error:
-    def __init__(self, message, line=None):
+    def __init__(self, message, line=None, details=None):
         self.message = message
         self.line = line
+        self.details = details
 
     def __repr__(self):
         s = self.message
@@ -687,9 +688,11 @@ class GitCommit:
         for file in sorted(mentioned_files - changed_files):
             msg = 'unchanged file mentioned in a ChangeLog'
             candidates = difflib.get_close_matches(file, changed_files, 1)
+            details = None
             if candidates:
                 msg += f' (did you mean "{candidates[0]}"?)'
-            self.errors.append(Error(msg, file))
+                details = '\n'.join(difflib.Differ().compare([file], [candidates[0]])).rstrip()
+            self.errors.append(Error(msg, file, details))
         for file in sorted(changed_files - mentioned_files):
             if not self.in_ignored_location(file):
                 if file in self.new_files: