Add "archive" as an alias for "archive-z2"

I find the "-z2" is really a long ago relic of the past when I changed
the format.  We no longer have anything to do with the original
`archive`, so let's start allowing people to type `--mode=archive`
which just looks saner.

At some point later I'll update the docs too, but it'll be an annoying
transition period as we'll have to say "On older OSTree, use -z2" etc.

Closes: #346
Approved by: giuseppe

src/libostree/ostree-repo.c

@@ -1816,7 +1816,8 @@ ostree_repo_mode_from_string (const char      *mode,
     ret_mode = OSTREE_REPO_MODE_BARE;
   else if (strcmp (mode, "bare-user") == 0)
     ret_mode = OSTREE_REPO_MODE_BARE_USER;
-  else if (strcmp (mode, "archive-z2") == 0)
+  else if (strcmp (mode, "archive-z2") == 0 ||
+           strcmp (mode, "archive") == 0)
     ret_mode = OSTREE_REPO_MODE_ARCHIVE_Z2;
   else
     {