Silent "no such file" when integration is not in progress

When 'git integration' is invoked with --continue or --abort but there
is no integration in progress, we attempt to get the contents of a
status file that does not exist.  This causes cat to print a "no such
file" error which presents the user with irrelevant implementation
details.  Suppress this.

Signed-off-by: John Keeping <john@keeping.me.uk>
Only apply auto-rebuild if something has changed

This will allow us to introduce interrogation commands and avoid
auto-rebuild to kicking in when no modification commands are given.

Signed-off-by: John Keeping <john@keeping.me.uk>
Set PS4 in debug mode

When GIT_INTEGRATION_DEBUG is specified, it's also useful to also have
the filename and line numbers with the trace.  Do this by also setting
PS4 when $GIT_INTEGRATION_DEBUG is non-empty.

Signed-off-by: John Keeping <john@keeping.me.uk>