RFR: 8042841 : hgforest.sh better exit code handling, clone options, verbose mode, options documentation

[Mike Duigou]

hello all;

This is a diverse set of changes that I have been working on for a while to improve various aspects of the hgforest command. 

It includes:

- Various minor cleanups
- Better documentation of options and important shell variables.
- Verbose output mode
- Further refinement of output redirection logic
* ability to support options and arguments to the clone command
- Uses -R rather than cd to select the target repo
- Better handling of exit codes. Exit code (1) is not flagged as an error.

The options and arguments for clone command is the driver for proposing the review now as there's a need to be able to clone specific revisions for bisection and regression testing.

jbsbug: https://bugs.openjdk.java.net/browse/JDK-8042841
webrev: http://cr.openjdk.java.net/~mduigou/JDK-8042841/0/webrev/

Mike