completion/subversion: load system completion

Load the completion script from the subversion package installed on the system, instead of bundling a copy. This addresses Bash-it#1818.

NOTE: If `completions/system` is enabled, then it will load this same file anyway automatically.

Author: gaelicWizard

completion/available/subversion.completion.bash

@@ -0,0 +1,7 @@
+# shellcheck shell=bash
+
+scm
+if test -s "${SVN_EXE%/*}/../etc/bash_completion.d/subversion"
+then
+	source "$_"
+fi