cli/rev-parse: Add --single option

In the current "ostree native container" flow, we're inserting
a commit object into the repo but with no refs.

We have hacks in a few places to find the commit digest via e.g.
`find repo/objects -name *.commit` but that's a horrible hack.
Add `ostree rev-parse --single` which will print the single commit,
and error out if there is not exactly one commit.

Co-authored-by: Jonathan Lebon <jonathan@jlebon.com>

src/ostree/ot-builtin-rev-parse.c

@@ -25,13 +25,17 @@
 #include "ot-builtins.h"
 #include "ostree.h"
 #include "otutil.h"
+#include <stdbool.h>
 
 /* ATTENTION:
  * Please remember to update the bash-completion script (bash/ostree) and
  * man page (man/ostree-rev-parse.xml) when changing the option list.
  */
 
+static gboolean opt_single;
+
 static GOptionEntry options[] = {
+  { "single", 'S', 0, G_OPTION_ARG_NONE, &opt_single, "If the repository has exactly one commit, then print it; any other case will result in an error", NULL },
   { NULL }
 };
 
@@ -43,11 +47,43 @@ ostree_builtin_rev_parse (int argc, char **argv, OstreeCommandInvocation *invoca
   if (!ostree_option_context_parse (context, options, &argc, &argv, invocation, &repo, cancellable, error))
     return FALSE;
 
+  if (opt_single)
+    {
+      if (argc >= 2)
+        {
+          ot_util_usage_error (context, "Cannot specify arguments with --single", error);
+          return FALSE;
+        }
+
+      g_autoptr(GHashTable) objects = NULL;
+      if (!ostree_repo_list_commit_objects_starting_with (repo, "", &objects, cancellable, error))
+        return FALSE;
+
+      GVariant *found = NULL;
+      GLNX_HASH_TABLE_FOREACH (objects, GVariant*, key)
+        {
+          if (found)
+            return glnx_throw (error, "Multiple commit objects found");
+          found = key;
+        }
+      if (!found)
+        return glnx_throw (error, "No commit objects found");
+      const char *checksum;
+      OstreeObjectType objtype;
+      ostree_object_name_deserialize (found, &checksum, &objtype);
+      g_assert (objtype == OSTREE_OBJECT_TYPE_COMMIT);
+
+      g_print ("%s\n", checksum);
+
+      return TRUE; /* Note early return */
+    }
+
   if (argc < 2)
     {
       ot_util_usage_error (context, "REV must be specified", error);
       return FALSE;
     }
+
   for (gint i = 1; i < argc; i++)
     {
       const char *rev = argv[i];