watch: exit when command output changes.

Add new flags to watch (-g/--chgexit) so that it exits when the
output changes.  This is useful in builds and shell scripts, for
example when deploying webapps to block the remainder of the
deployment steps until after the webapp starts.

e.g. watch --chgexit curl http://foo/bar

watch.1

@@ -11,6 +11,7 @@ runs
 repeatedly, displaying its output and errors (the first screenfull).  This
 allows you to watch the program output change over time.  By default, the
 program is run every 2 seconds.
+By default,
 .B watch
 will run until interrupted.
 .SH OPTIONS
@@ -44,6 +45,11 @@ Beep if command has a non-zero exit.
 \fB\-e\fR, \fB\-\-errexit\fR
 Freeze updates on command error, and exit after a key press.
 .TP
+\fB\-g\fR, \fB\-\-chgexit\fR
+Exit when the output of
+.I command
+changes.
+.TP
 \fB\-c\fR, \fB\-\-color\fR
 Interpret ANSI color sequences.
 .TP