Explain seemingly pointless use of system in difftool

Portability reasons.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -82,5 +82,11 @@ sub generate_command
 }
 
 setup_environment();
+
+# ActiveState Perl for Win32 does not implement POSIX semantics of
+# exec* system call. It just spawns the given executable and finishes
+# the starting program, exiting with code 0.
+# system will at least catch the errors returned by git diff,
+# allowing the caller of git difftool better handling of failures.
 my $rc = system(generate_command());
 exit($rc | ($rc >> 8));