Revert "gcc-changelog: temporarily disable check_line_start"

This reverts commit a5878983d53db7513edb0e6bd99250dbb0c7d147.
diff --git a/contrib/gcc-changelog/git_commit.py b/contrib/gcc-changelog/git_commit.py
index 59f9680..3bd6710 100755
--- a/contrib/gcc-changelog/git_commit.py
+++ b/contrib/gcc-changelog/git_commit.py
@@ -615,8 +615,6 @@
                 self.errors.append(Error(msg, entry.parentheses_stack[-1]))
 
     def check_line_start(self):
-        # FIXME: temporarily disable
-        return
         for entry in self.changelog_entries:
             for line in entry.lines:
                 if line.startswith('\t '):