Special handling for manifest group "default"

Change Details:
* Make "default" a special manifest group that matches any project that
  does not have the special project group "notdefault"
* Use "default" instead of "all,-notdefault" when user does not specify
  manifest group
* Expand -g option help to include example usage of manifest groups

Change Benefits:
* Allow a more intuitive and expressive manifest groups specification:
  * "default" instead of "all,-notdefault"
  * "default,foo" instead of "all,-notdefault,foo"
  * "default,-foo" instead of "all,-notdefault,-foo"
  * "foo,-default" which has no equivalent
* Default manifest groups behavior can be restored by the command
  'repo init -g default'. This is significantly more intuitive than the
  current equivalent command 'repo init -g all,-notdefault'.

Change-Id: I6d0673791d64a650110a917c248bcebb23b279d3
4 files changed