libgit2: use peel() in the pygit2 example

get_object() is deprecated, and we now have the common peeling mechanism
which we can use to get to the commit more elegantly.
Fix missed include

Looks like this include is missed, as I see reference error in http://git-scm.com/book/en/v2/Git-Commands-Sharing-and-Updating-Projects#git-push
Merge pull request #159 from carlosmn/pygit2-peel

libgit2: use peel() in the pygit2 example
Merge pull request #160 from madhead/patch-2

Fix missed include
Fix name of the submodule entry.

Was `rack` in 1st edition but now it's `DbConnector`.
Clean full prompt
Clean doubled `f` option
Revert "Fix missed include"
Merge pull request #162 from progit/revert-160-patch-2

Revert "Fix missed include"
Merge pull request #161 from zigarn/fixes

Some simple fixes