Fix git cl completion

__git_cl_all_commands is a variable, so it should use the variable
parameter, ${}, not a command substitution $().

Change-Id: I81fc2c73afd85cc1f737af00fbc294d933145e22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2205456
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cl_completion.sh

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