contrib/difftool: remove distracting 'echo' in the SIGINT handler

When interrupting git-difftool with Ctrl-C, the output of this echo
command led to having the cursor at the beginning of the line below the
shell prompt.
Signed-off-by: NMarkus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/difftool/git-difftool-helper

@@ -32,7 +32,6 @@ cleanup_temp_files () {
 
 # This is called when users Ctrl-C out of git-difftool-helper
 sigint_handler () {
-	echo
 	cleanup_temp_files
 	exit 1
 }