Update 'project sync' with '--source' to generate a PR from 'source' remote updates.

[zanerock]

Overview

With the new 'create' setup, it's possible to pickup a 'source' repo. If '--followed', then syncing should update the upstream remote from the source remote. I.e.:

1. source/master -> (if it exists)
2. upstream/master ->
3. workspace/master ->
4. workspace/<work branch> (if it exists)

Deliverables

• Add --source option to projects sync
• --source option is unitary; it cannot be combined with other options.
• If --source option included, check if a source remote is available.
  • If no, then echoerrandexit with "No 'source' remote found."
  • If yes, then checkout a source/master working tree,
  • Pull changes from source/master remote,
  • Create work unit branch with description "source sync"
  • Merge source/master into newly created workbranch
  • If merge fails, echoerrandexit with "Failed to merge updates form 'source/master'. Please complete merge and execute: liq work submit"
  • If merge successful, submit PR.