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/bash-it#1818.

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

completion/available/subversion.completion.bash

@@ -0,0 +1,40 @@
+# shellcheck shell=bash
+#
+# Locate and load completions for `svn`.
+
+# Make sure svn is installed
+_command_exists svn || return
+
+# Don't handle completion if it's already managed
+if _completion_exists svn; then
+	_log_warning "completion already loaded - this usually means it is safe to stop using this completion"
+	return 0
+fi
+
+_svn_bash_completion_xcrun_svn=
+if _command_exists xcrun; then
+	_svn_bash_completion_xcrun_svn="$(xcrun --find svn)"
+fi
+_svn_bash_completion_paths=(
+	# Standard locations
+	"${SVN_EXE%/*}/../etc/bash_completion.d/subversion"
+	# MacOS non-system locations
+	"${_svn_bash_completion_xcrun_svn%/bin/svn}/etc/bash_completion.d/subversion"
+)
+
+# Load the first completion file found
+_svn_bash_completion_found=false
+for _comp_path in "${_svn_bash_completion_paths[@]}"; do
+	if [[ -r "$_comp_path" ]]; then
+		_svn_bash_completion_found=true
+		# shellcheck disable=SC1090 # don't follow
+		source "$_comp_path"
+		break
+	fi
+done
+
+# Cleanup
+if [[ "${_svn_bash_completion_found}" == false ]]; then
+	_log_warning "no completion files found - please try enabling the 'system' completion instead."
+fi
+unset "${!_svn_bash_completion@}"