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, 7 insertions, 2 deletions

diff --git a/include/git2/commit.h b/include/git2/commit.h
index a420ba635..20b345f84 100644
--- a/include/git2/commit.h
+++ b/include/git2/commit.h
@@ -164,7 +164,10 @@ GIT_EXTERN(unsigned int) git_commit_parentcount(const git_commit *commit);
  * @param n the position of the parent (from 0 to `parentcount`)
  * @return 0 or an error code
  */
-GIT_EXTERN(int) git_commit_parent(git_commit **out, git_commit *commit, unsigned int n);
+GIT_EXTERN(int) git_commit_parent(
+	git_commit **out,
+	const git_commit *commit,
+	unsigned int n);
 
 /**
  * Get the oid of a specified parent for a commit. This is different from
@@ -175,7 +178,9 @@ GIT_EXTERN(int) git_commit_parent(git_commit **out, git_commit *commit, unsigned
  * @param n the position of the parent (from 0 to `parentcount`)
  * @return the id of the parent, NULL on error.
  */
-GIT_EXTERN(const git_oid *) git_commit_parent_id(git_commit *commit, unsigned int n);
+GIT_EXTERN(const git_oid *) git_commit_parent_id(
+	const git_commit *commit,
+	unsigned int n);
 
 /**
  * Get the commit object that is the <n>th generation ancestor