Completion must sort before using uniq

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that completion is no longer valid.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>
Reviewed-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>