Add cat-file example and increase const use in API

This adds an example implementation that emulates git cat-file.
It is a convenient and relatively simple example of getting data
out of a repository.

Implementing this also revealed that there are a number of APIs
that are still not using const pointers to objects that really
ought to be.  The main cause of this is that `git_vector_bsearch`
may need to call `git_vector_sort` before doing the search, so a
const pointer to the vector is not allowed.  However, for tree
objects, with a little care, we can ensure that the vector of
tree entries is always sorted and allow lookups to take a const
pointer.  Also, the missing const in commit objects just looks
like an oversight.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/commit.c b/src/commit.c
index be6e32c76..1ab9b34f7 100644
--- a/src/commit.c
+++ b/src/commit.c
@@ -266,14 +266,16 @@ int git_commit_tree(git_tree **tree_out, const git_commit *commit)
 	return git_tree_lookup(tree_out, commit->object.repo, &commit->tree_id);
 }
 
-const git_oid *git_commit_parent_id(git_commit *commit, unsigned int n)
+const git_oid *git_commit_parent_id(
+	const git_commit *commit, unsigned int n)
 {
 	assert(commit);
 
 	return git_vector_get(&commit->parent_ids, n);
 }
 
-int git_commit_parent(git_commit **parent, git_commit *commit, unsigned int n)
+int git_commit_parent(
+	git_commit **parent, const git_commit *commit, unsigned int n)
 {
 	const git_oid *parent_id;
 	assert(commit);