diff-highlight: exit when a pipe is broken

While using diff-highlight with other tools, I have discovered that Python
ignores SIGPIPE by default.  Unfortunately, this also means that tools
attempting to launch a pager under Python--and don't realize this is
happening--means that the subprocess inherits this setting.  In this case, it
means diff-highlight will be launched with SIGPIPE being ignored.  Let's work
with those broken scripts by explicitly setting up a SIGPIPE handler and exiting
the process.