core: Make OSTREE_TIMESTAMP public API

This way e.g. flatpak can detect which timestamp it should use.
See `flatpak/common/flatpak-utils.c:flatpak_zero_mtime`.

Closes: #501
Approved by: cgwalters

src/libostree/ostree-core.h

@@ -165,6 +165,15 @@ typedef enum {
 #define OSTREE_SUMMARY_SIG_GVARIANT_STRING "a{sv}"
 #define OSTREE_SUMMARY_SIG_GVARIANT_FORMAT G_VARIANT_TYPE (OSTREE_SUMMARY_SIG_GVARIANT_STRING)
 
+/**
+ * OSTREE_TIMESTAMP:
+ *
+ * The mtime used for stored files.  This was originally 0, changed to 1 for
+ * a few releases, then was reverted due to regressions it introduced from
+ * users who had been using zero before.
+ */
+#define OSTREE_TIMESTAMP (0)
+
 /**
  * OstreeRepoMode:
  * @OSTREE_REPO_MODE_BARE: Files are stored as themselves; checkouts are hardlinks; can only be written as root