pull: Support specifying exact commit to pull via branch@commit

I don't know why we didn't do this a long time ago.  This extends the
pull API to allow grabbing a specific commit, and will set the branch
to it.  There's some support for this in the deploy engine, but there
are a lot of reasons to support it for raw pulls (such as subset
mirroring cases).

In fact I'm thinking we should also have the override-version logic
here too.

NOTE: One thing I debated here is inventing a new syntax on the
command line.  Git doesn't seem to have this functionality (probably
because it'd be rarely used). The '@' character at least doesn't
conflict with anything.

Anyways, I wanted this for some other test cases.  Without this,
writing tests that go between different commits is more awkward as one
must generate the content in one repo, then pull downstream, then
generate more content, then pull again.  But now I can just keep track
of commit IDs and do exactly what I want without synchronizing the
tests.

src/libostree/ostree-repo-pull.c

@@ -1762,6 +1762,7 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
   OstreeRepoPullFlags flags = 0;
   const char *dir_to_pull = NULL;
   char **refs_to_fetch = NULL;
+  char **override_commit_ids = NULL;
   GSource *update_timeout = NULL;
   gboolean disable_static_deltas = FALSE;
 
@@ -1776,9 +1777,12 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
       (void) g_variant_lookup (options, "override-remote-name", "s", &pull_data->remote_name);
       (void) g_variant_lookup (options, "depth", "i", &pull_data->maxdepth);
       (void) g_variant_lookup (options, "disable-static-deltas", "b", &disable_static_deltas);
+      (void) g_variant_lookup (options, "override-commit-ids", "^a&s", &override_commit_ids);
     }
 
   g_return_val_if_fail (pull_data->maxdepth >= -1, FALSE);
+  if (refs_to_fetch && override_commit_ids)
+    g_return_val_if_fail (g_strv_length (refs_to_fetch) == g_strv_length (override_commit_ids), FALSE);
 
   if (dir_to_pull)
     g_return_val_if_fail (dir_to_pull[0] == '/', FALSE);
@@ -2069,8 +2073,10 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
     } 
   else if (refs_to_fetch != NULL)
     {
-      char **strviter;
-      for (strviter = refs_to_fetch; *strviter; strviter++)
+      char **strviter = refs_to_fetch;
+      char **commitid_strviter = override_commit_ids ? override_commit_ids : NULL;
+
+      while (*strviter)
         {
           const char *branch = *strviter;
 
@@ -2081,8 +2087,13 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
             }
           else
             {
-              g_hash_table_insert (requested_refs_to_fetch, g_strdup (branch), NULL);
+              char *commitid = commitid_strviter ? g_strdup (*commitid_strviter) : NULL;
+              g_hash_table_insert (requested_refs_to_fetch, g_strdup (branch), commitid);
             }
+          
+          strviter++;
+          if (commitid_strviter)
+            commitid_strviter++;
         }
     }
   else
@@ -2109,28 +2120,36 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
   while (g_hash_table_iter_next (&hash_iter, &key, &value))
     {
       const char *branch = key;
+      const char *override_commitid = value;
       char *contents = NULL;
 
-      if (pull_data->summary)
+      /* Support specifying "" for an override commitid */
+      if (override_commitid && *override_commitid)
         {
-          gsize commit_size = 0;
-          guint64 *malloced_size;
-
-          if (!lookup_commit_checksum_from_summary (pull_data, branch, &contents, &commit_size, error))
-            goto out;
-
-          malloced_size = g_new0 (guint64, 1);
-          *malloced_size = commit_size;
-          g_hash_table_insert (pull_data->expected_commit_sizes, g_strdup (contents), malloced_size);
+          g_hash_table_replace (requested_refs_to_fetch, g_strdup (branch), g_strdup (override_commitid));
         }
-      else
+      else    
         {
-          if (!fetch_ref_contents (pull_data, branch, &contents, cancellable, error))
-            goto out;
+          if (pull_data->summary)
+            {
+              gsize commit_size = 0;
+              guint64 *malloced_size;
+
+              if (!lookup_commit_checksum_from_summary (pull_data, branch, &contents, &commit_size, error))
+                goto out;
+
+              malloced_size = g_new0 (guint64, 1);
+              *malloced_size = commit_size;
+              g_hash_table_insert (pull_data->expected_commit_sizes, g_strdup (contents), malloced_size);
+            }
+          else
+            {
+              if (!fetch_ref_contents (pull_data, branch, &contents, cancellable, error))
+                goto out;
+            }
+          /* Transfer ownership of contents */
+          g_hash_table_replace (requested_refs_to_fetch, g_strdup (branch), contents);
         }
-      
-      /* Transfer ownership of contents */
-      g_hash_table_replace (requested_refs_to_fetch, g_strdup (branch), contents);
     }
 
   /* Create the state directory here - it's new with the commitpartial code,