This patch fixes the issue where our command-line tab completer would sometimes replicate commands
e.g.

(lldb) pl<TAB>
Available completions:
	platform
	plugin
	platform
	plugin

Thanks to Matthew Sorrels for doing work and testing on this issue

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@184212 91177308-0d34-0410-b5e6-96231b3b80d8