upgrade: Add support for --pull-only and --deploy-only

This makes it easier to script downloading updates in the background,
and only do deployments just before rebooting.

Partially addresses https://github.com/ostreedev/ostree/issues/640

Closes: #642
Approved by: jlebon

src/libostree/ostree-sysroot-upgrader.h

@@ -85,7 +85,8 @@ gboolean ostree_sysroot_upgrader_check_timestamps (OstreeRepo     *repo,
 
 typedef enum {
   OSTREE_SYSROOT_UPGRADER_PULL_FLAGS_NONE = 0,
-  OSTREE_SYSROOT_UPGRADER_PULL_FLAGS_ALLOW_OLDER = (1 << 0)
+  OSTREE_SYSROOT_UPGRADER_PULL_FLAGS_ALLOW_OLDER = (1 << 0),
+  OSTREE_SYSROOT_UPGRADER_PULL_FLAGS_SYNTHETIC = (1 << 1) /* Don't actually do a pull, just check timestamps/changed */
 } OstreeSysrootUpgraderPullFlags;
 
 _OSTREE_PUBLIC