Merge branch 'sg/completion-clear-cached'

The completion script (in contrib/) learned to clear cached list of
command line options upon dot-sourcing it again in a more efficient
way.

* sg/completion-clear-cached:
  completion: reduce overhead of clearing cached --options

contrib/completion/git-completion.bash

@@ -284,7 +284,11 @@ __gitcomp ()
 
 # Clear the variables caching builtins' options when (re-)sourcing
 # the completion script.
-unset $(set |sed -ne 's/^\(__gitcomp_builtin_[a-zA-Z0-9_][a-zA-Z0-9_]*\)=.*/\1/p') 2>/dev/null
+if [[ -n ${ZSH_VERSION-} ]]; then
+	unset $(set |sed -ne 's/^\(__gitcomp_builtin_[a-zA-Z0-9_][a-zA-Z0-9_]*\)=.*/\1/p') 2>/dev/null
+else
+	unset $(compgen -v __gitcomp_builtin_)
+fi
 
 # This function is equivalent to
 #