ignore test.*.diff

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
sanitize environment variables

ctags invokes sort command via system function.  At least on some
GNU/Linux distribution uses bash as default command interpreter.

This patch tries to decrease the impact ShellShock vulnerability.
(Of course the best way is updating bash. )

If value for an environment variable starts with '() {',
clear the value. When clearing, ctags warns it like:

    $ X='() {echo xxx;}' ./ctags
    X='() {echo xxx;}' ./ctags
    ctags: Warning: reset environment: X=() {echo xxx;}

Signed-off-by: Masatake YAMATO <yamato@redhat.com>