libgit2: use set_ob() instead of wrap_odb() for a custom backend

The purpose of git_repository_warp_odb() is not to use a repository with
a custom odb backend, but to access an odb via functions which want a
repository object. A repository build this way has no path, no refs db,
no configuration, no worktree, etc. It is useless unless you're
e.g. trying to extract information out of a packfile via
git_object_lookup() and friends.

In order to have an otherwise "normal" repository but use a custom
backend for the objects, you need to open/init the repository as you
usually do and then set the custom odb.

Author: carlosmn

book/B-embedding-git/sections/libgit2.asc

@@ -124,24 +124,25 @@ Here's how a custom backend for the object database is set up:
 git_odb *odb;
 int error = git_odb_new(&odb); // <1>
 
-git_repository *repo;
-error = git_repository_wrap_odb(&repo, odb); // <2>
-
 git_odb_backend *my_backend;
-error = git_odb_backend_mine(&my_backend, /*…*/); // <3>
+error = git_odb_backend_mine(&my_backend, /*…*/); // <2>
 
-error = git_odb_add_backendodb, my_backend, 1); // <4>
+error = git_odb_add_backendodb, my_backend, 1); // <3>
+
+git_repository *repo;
+error = git_repository_open(&repo, "some-path");
+error = git_repository_set_odb(odb); // <4>
 ----
 
 _(Note that errors are captured, but not handled. We hope your code is better than ours.)_
 
-<1> Initialize an empty object database (ODB) ``frontend,'' which will act as a handle to the real ODB.
-<2> Construct a `git_repository` around the empty ODB.
-<3> Initialize a custom ODB backend.
-<4> Set the repository to use the custom backend for its ODB.
+<1> Initialize an empty object database (ODB) ``frontend,'' which will act as a container for the ``backends'' which are the ones doing the real work.
+<2> Initialize a custom ODB backend.
+<3> Open a repository
+<4> Set the repository to use our ODB to look up objects.
 
 But what is this `git_odb_backend_mine` thing?
-Well, that's your own ODB implementation, and you can do whatever you want in there, so long as you fill in the `git_odb_backend` structure properly.
+Well, that's the constructor for your own ODB implementation, and you can do whatever you want in there, so long as you fill in the `git_odb_backend` structure properly.
 Here's what it _could_ look like:
 
 [source,c]