lib/deploy: Log case when auto-pruning is hopeless

For easier diagnostics.

src/libostree/ostree-sysroot-deploy.c

@@ -2578,6 +2578,7 @@ auto_early_prune_old_deployments (OstreeSysroot *self, GPtrArray *new_deployment
     {
       /* Even if we auto-pruned, the new bootdirs wouldn't fit. Just let the
        * code continue and let it hit ENOSPC. */
+      g_printerr ("Disabling auto-prune optimization; insufficient space left in bootfs\n");
       return TRUE;
     }
 

tests/kolainst/destructive/auto-prune.sh

@@ -46,6 +46,7 @@ rpm-ostree rebase :modkernel1
 if OSTREE_SYSROOT_OPTS=early-prune ostree admin finalize-staged |& tee out.txt; then
     assert_not_reached "successfully wrote to filled up bootfs"
 fi
+assert_file_has_content out.txt "Disabling auto-prune optimization; insufficient space left in bootfs"
 assert_file_has_content out.txt "No space left on device"
 rm out.txt
 unconsume_bootfs_space