sysroot: Support more arbitrary deployment changes

This commit changes the sysroot API so that one can create arbitrary
new deployment checkouts, then commit them as one step.  This is to
enable things like an automatic bisection tool which say create 50
deployments at once, then when done clean them up.

This also moves some printfs from the library into src/ostree.

src/libostree/ostree-sysroot.c

@@ -1070,6 +1070,10 @@ ostree_sysroot_get_merge_deployment (OstreeSysroot     *self,
   if (osname == NULL)
     osname = ostree_deployment_get_osname (self->booted_deployment);
 
+  /* If we're booted into the OS into which we're deploying, then
+   * merge the currently *booted* configuration, rather than the most
+   * recently deployed.
+   */
   if (self->booted_deployment &&
       g_strcmp0 (ostree_deployment_get_osname (self->booted_deployment), osname) == 0)
     {