Improve usage of some rbt land options.
Review Request #6723 — Created Jan. 2, 2015 and submitted
This adds short flags for the --push (-p) and --no-push (-n) options, which are likely to be some of the more commonly-used options. It adds a configuration key to set the default for LAND_SQUASH, and adds a --no-squash option to override if set. It also adds a missing period in the help text for --squash.
Tested --squash and --no-squash.
Tested with -p and -n.