Support simple comments in the bookmarks file

Put it in a better place...

Accessed by _load_config_file()

Fix duplicate detection in case agnostic systems

specify config file delimiter so it doesn't barf on windows paths

* Only update _fast-forwardable_ branches to their upstreams. This uses
  `git merge --no-ff` for the active branch and `git branch --force`
  for others. Checks for clean working directories and ancestry are
  performed as expected.
* Added --fetch-only / -f to disable branch updating entirely, if
  desired.
* Deprecated --merge and --rebase. These have no effect now and instead
  give a warning.