delta: fix warnings, fix for non-GNU date tool

[vszakats]

It makes the script run on BSD-like envs.

Follow-up to f63bdea #18058
Follow-up to 2ec5455 #17877

[bagder]

This commit broke several of the counters

[vszakats]

Sorry! I think it's the wrong scope of mode. Fix coming up → #18074.

[vszakats]

Does this fix it?:

--- a/scripts/delta
+++ b/scripts/delta
@@ -119,9 +119,9 @@ if($diffstat =~ /^ *(\d+) files changed, (\d+) insertions\(\+\), (\d+)/) {
 my $numchanges = 0;
 my $numbugfixes = 0;
 my $numcontributors = 0;
+my $mode = 0;
 open(F, "<RELEASE-NOTES");
 while(<F>) {
-    my $mode=0;
     if($_ =~ /following changes:/) {
         $mode=1;
     }