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Autocompletion fails if there's only one mark, since the ls command
will not display the parent directory with the trailing colon.
Handling the single mark case separately and validating the symlink
explicitly, resolves the issue.

Change marks function and remove 'function' keyword as suggested by pielgrzym in https://github.com/robbyrussell/oh-my-zsh/pull/2045#issuecomment-22820224

Add _completemarks function as suggested by pielgrzym in https://github.com/robbyrussell/oh-my-zsh/pull/2045#issuecomment-22826540

The typical command is `subl`, not `st`. Leaving both for backward compatibility.

See http://www.sublimetext.com/docs/2/osx_command_line.html

Added taps to bundled_commands

* -r, --record now looks for a local file instead of trying to cache
  the entire package index
* add --editable because it's useful
* add --single-version-externally-managed because it is too long
* add --record and --root because they're required by
  --single-version-externally-managed

Copy and paste of two functions from Ubuntu 13.04's version of /usr/share/zsh/functions/Completion/Unix/_git that were referenced in 46f0d8db. Fixes #1952.