Use cached variable for git cl completion

__git_cl_all_commands is used to cache the result of
__git_cl_commands(), but the actual code wasn't using it and was calling
the function everytime instead.

Change-Id: Ia2ce32b7de2812349808ec9aceeacf5adf463700
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2190337
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>

git_cl_completion.sh

@@ -30,5 +30,5 @@ __git_cl_compute_all_commands () {
 
 _git_cl () {
   __git_cl_compute_all_commands
-  __gitcomp_nl "$(__git_cl_commands)"
+  __gitcomp_nl "$(__git_cl_all_commands)"
 }