This patch adjusts the 'gitlog-to-changelog' script from Gnulib so
that its output is reproducible.

diff --git a/build-aux/gitlog-to-changelog b/build-aux/gitlog-to-changelog
index deddef2..6fec950 100755
--- a/build-aux/gitlog-to-changelog
+++ b/build-aux/gitlog-to-changelog
@@ -345,7 +345,7 @@ sub git_dir_option($)
                   ? '  (tiny change)' : '');
 
       my $date_line = sprintf "%s  %s$tiny\n",
-        strftime ("%Y-%m-%d", localtime ($1)), $2;
+        strftime ("%Y-%m-%d", gmtime ($1)), $2;
 
       my @coauthors = grep /^Co-authored-by:.*$/, @line;
       # Omit meta-data lines we've already interpreted.