bash_completion: support GNU sed 3 and non-GNU sed replaced command 'Q' introduced in GNU sed 4, in order to support platforms with older sed.

diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123