Increase git_changelog's timestamp_slop from 10 min to 1 day.

Many committers seem to now be using a work flow in which back-patched
commits are timestamped minutes or even hours apart in different branches
(most likely because they commit in one branch before starting work on
the next one).  git_changelog was failing to merge its reports in such
cases, so increase the max time it's willing to merge commits across.
I considered getting rid of the limit altogether, but that produces
some odd results in terms of how the merged commit gets sorted relative
to unrelated commits.

src/tools/git_changelog

@@ -44,7 +44,7 @@ my @BRANCHES = qw(master
   REL6_5_PATCHES REL6_4);
 
 # Might want to make this parameter user-settable.
-my $timestamp_slop = 600;
+my $timestamp_slop = 24*60*60;
 
 my $details_after = 0;
 my $post_date     = 0;