lib: Coerce flags enums to GIR bitfields

The GI scanner decides if an `enum` is really a `bitfield` if it finds
any values that have left shifts. With an `enumeration`, the
introspecting language may error or convert to a different type if the
user tries to combine values. Change all Flags `enum`s to use
left-shifted values so that they're represented as `bitfield`s in the
GIR.

The primary bug here is that you can't combine `REFS_ONLY` and
`NO_PRUNE` when calling `OSTree.Repo.prune()` from an introspected
language.

This is an IABI break since the typelib will change from `enumeration`
to `bitfield`. `OstreeRepoImportFlags` is internal but the change is
included here to prepare for a subsequent name that would require bit
shifting to operate correctly as a flag.

src/libostree/ostree-repo-private.h

@@ -374,9 +374,9 @@ _ostree_repo_verify_commit_internal (OstreeRepo    *self,
 #endif /* OSTREE_DISABLE_GPGME */
 
 typedef enum {
-  _OSTREE_REPO_IMPORT_FLAGS_NONE,
-  _OSTREE_REPO_IMPORT_FLAGS_TRUSTED,
-  _OSTREE_REPO_IMPORT_FLAGS_VERIFY_BAREUSERONLY,
+  _OSTREE_REPO_IMPORT_FLAGS_NONE = 0,
+  _OSTREE_REPO_IMPORT_FLAGS_TRUSTED = (1 << 0),
+  _OSTREE_REPO_IMPORT_FLAGS_VERIFY_BAREUSERONLY = (1 << 1),
 } OstreeRepoImportFlags;
 
 gboolean