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Force the mark to point to the new dir, replacing the old one.

Fixes #7195

Using the `jump` plugin, using the `marks` command will yield this output:
```
$ marks
desktop marks:printf:5: bad option: ->
dotfiles        marks:printf:5: bad option: ->
home    marks:printf:5: bad option: ->
```

the `marks` function uses `printf` with `->` and I believe `-` is used by `printf` for left-justification.  changing this to `-- "->"` seems to render the appropriate output.

```
desktop -> /Users/uname/Desktop
dotfiles        -> /Users/uname/.dotfiles
home    -> /Users/uname
```

This allows for the user to combine the jump command with something else. In my example cd and jump are now combined like this:

```bash
jumpcd() {
        jump $1 > /dev/null || cd $1
}
alias cd="jumpcd"
```

This avoids spawning additional processes as $PWD **always** contains
the current working directory.

This change fixes the autocompletion for marks which contain numbers or dots.

Fixes #2578

`\d` in sed regular expressions doesn't work (see http://stackoverflow.com/questions/14671293/why-does-d-doesnt-work-in-regular-expression-in-sed)

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