Turn down tracing for source-repo cache

If you check logs like [1] it's literally thousands of lines of the
same thing over-and-over as the git caching happens.  It is basically
all just noise unless you're debugging it specifically.  Up this to
tracing level 2 ("-x -x") to see it.  Add a note in the help about
multiple flags, which has always been intended but not documented.

Image builds should continue to run with single "-x", but we could
probably greatly increase signal:noise ratio in the logs with a little
more judicial use of this to turn down some of the very noisy &
repetitive parts.

[1] anything in http://nodepool.openstack.org/

Change-Id: I91c5e55814ba9840769357261d203f4850e2eba6

bin/disk-image-create

@@ -106,7 +106,7 @@ function show_options () {
     echo "       File types should be comma separated. VHD outputting requires the vhd-util"
     echo "       executable be in your PATH. ACI outputting requires the ACI_MANIFEST "
     echo "       environment variable be a path to a manifest file."
-    echo "    -x -- turn on tracing"
+    echo "    -x -- turn on tracing (use -x -x for very detailed tracing)"
     echo "    -u -- uncompressed; do not compress the image - larger but faster"
     echo "    -c -- clear environment before starting work"
     echo "    --image-size size -- image size in GB for the created image"

elements/source-repositories/extra-data.d/98-source-repositories

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+if [ ${DIB_DEBUG_TRACE:-0} -gt 1 ]; then
     set -x
 fi
 set -eu