Re: [PATCH v3] Completion must sort before using uniq

> The user can be presented with invalid completion results
> when trying to complete a 'git checkout' command.  This can happen
> when using a branch name prefix that matches multiple remote branches.
>
> For example, if available branches are:
>   master
>   remotes/GitHub/maint
>   remotes/GitHub/master
>   remotes/origin/maint
>   remotes/origin/master
>
> When performing completion on 'git checkout ma' the user will be
> given the choices:
>   maint
>   master
>
> However, 'git checkout maint' will fail in this case, although
> completion previously said 'maint' was valid.  Furthermore, when
> performing completion on 'git checkout mai', no choices will be
> suggested.  So, the user is first told that the branch name
> 'maint' is valid, but when trying to complete 'mai' into 'maint',
> that completion is no longer valid.
>
> The completion results should never propose 'maint' as a valid
> branch name, since 'git checkout' will refuse it.
>
> The reason for this bug is that the uniq program only
> works with sorted input.  The man page states
> "uniq prints the unique lines in a sorted file".
>
> When __git_refs uses the guess heuristic employed by checkout for
> tracking branches it wants to consider remote branches but only if
> the branch name is unique.  To do that, it calls 'uniq -u'.  However
> the input given to 'uniq -u' is not sorted.
>
> Therefore, in the above example, when dealing with 'git checkout ma',
> "__git_refs '' 1" will find the following list:
>   master
>   maint
>   master
>   maint
>   master
>
> which, when passed to 'uniq -u' will remain the same.  Therefore
> 'maint' will be wrongly suggested as a valid option.
>
> When dealing with 'git checkout mai', the list will be:
>   maint
>   maint
>
> which happens to be sorted and will be emptied by 'uniq -u',
> properly ignoring 'maint'.
>
> A solution for preventing the completion script from suggesting
> such invalid branch names is to first call 'sort' and then 'uniq -u'.
>
> Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>