updateAUTHORS.pm: overrule 'diff.algorithm'

When the 'diff.algorithm' in git config is set to 'patience'
(i.e. `git config diff.algorithm patience`) then t/porting/update_authors.t
failed because it can renders a slightly different diff which results in
different line counts which caused the test failed.

An example of such a commit f7e7b4d:
	$ git show --oneline --numstat --diff-algorithm=myers f7e7b4d
	f7e7b4d win32.c: use _mkgmtime() instead of mktime() in stat()
	11      2       pod/perldelta.pod
	10      13      win32/win32.c

	$ git show --oneline --numstat --diff-algorithm=patience f7e7b4d
	f7e7b4d win32.c: use _mkgmtime() instead of mktime() in stat()
	11      2       pod/perldelta.pod
	9       12      win32/win32.c

Note: I opted to use `git -c diff.algorithm` since that will also
      work on old git versions that don't support `git log --diff-algorithm`

Porting/updateAUTHORS.pm

@@ -211,7 +211,7 @@ sub read_commit_log {
     push @args, "'$self->{commit_range}'" if $self->{commit_range};
 
     my $last_commit_info;
-    my $cmd= qq(git log @args);
+    my $cmd= qq(git -c diff.algorithm=myers log @args);
     $cmd =~ s/'/"/g if $^O =~ /Win/;
     open my $fh, "-|", $cmd
         or die "Failed to open git log pipe: $!";