documentation: add brief mention of cat-file to tutorial part I

diff --git a/merge-base.c b/merge-base.c
index 07f5ab4d1c72afeac5e3f193cefe91801ee9a9c9..4856ca01c33896843c366f3fb5edadd2ea3ced37 100644 (file)
--- a/merge-base.c
+++ b/merge-base.c
@@ -82,8 +82,9 @@ static struct commit *interesting(struct commit_list *list)
  * commit B.
  *
  *
- * Another pathological example how this thing can fail to mark an ancestor
- * of a merge base as UNINTERESTING without the postprocessing phase.
+ * Another pathological example how this thing used to fail to mark an
+ * ancestor of a merge base as UNINTERESTING before we introduced the
+ * postprocessing phase (mark_reachable_commits).
  *
  *               2
  *               H
@@ -118,7 +119,9 @@ static struct commit *interesting(struct commit_list *list)
  *      D7                     2 3 7 7 3 2 1 2
  *      E7                     2 3 7 7 7 2 1 2
  *
- * and we end up showing E as an interesting merge base.
+ * and we ended up showing E as an interesting merge base.
+ * The postprocessing phase re-injects C and continues traversal
+ * to contaminate D and E.
  */
 
 static int show_all = 0;
@@ -247,10 +250,12 @@ int main(int argc, char **argv)
                        usage(merge_base_usage);
                argc--; argv++;
        }
-       if (argc != 3 ||
-           get_sha1(argv[1], rev1key) ||
-           get_sha1(argv[2], rev2key))
+       if (argc != 3)
                usage(merge_base_usage);
+       if (get_sha1(argv[1], rev1key))
+               die("Not a valid object name %s", argv[1]);
+       if (get_sha1(argv[2], rev2key))
+               die("Not a valid object name %s", argv[2]);
        rev1 = lookup_commit_reference(rev1key);
        rev2 = lookup_commit_reference(rev2key);
        if (!rev1 || !rev2)