core: Associate branches with remotes, move trigger runs into checkout

Also add --atomic-retarget option to checkout. This does the magical
symlink dance to do atomic swaps between trees.

src/libostree/README.md

@@ -47,6 +47,7 @@ MILESTONE 2
 * Add index size to superindex, pack size to index
   - So pull can calculate how much we need to download
 * Split pack files into metadata/data
+* pull: Extract all we can from each packfile one at a time, then delete it
 * Restructure repository so that links can be generated as a cache;
   i.e. objects/raw, pack files are now the canonical
 * For files, checksum combination of metadata variant + raw data