annotated_commit: provide a constructor from a revspec

This extra constructor will be useful for the annotated versions of
ref-modifying functions, as it allows us to create a commit with the
extended sha syntax which was used to retrieve it.

include/git2/annotated_commit.h

@@ -78,6 +78,23 @@ GIT_EXTERN(int) git_annotated_commit_lookup(
 	const git_oid *id);
 
 /**
+ * Creates a `git_annotated_comit` from a revision string.
+ *
+ * See `man gitrevisions`, or
+ * http://git-scm.com/docs/git-rev-parse.html#_specifying_revisions for
+ * information on the syntax accepted.
+ *
+ * @param out pointer to store the git_annotated_commit result in
+ * @param repo repository that contains the given commit
+ * @param revspec the extended sha syntax string to use to lookup the commit
+ * @return 0 on success or error code
+ */
+GIT_EXTERN(int) git_annotated_commit_from_revspec(
+	git_annotated_commit **out,
+	git_repository *repo,
+	const char *revspec);
+
+/**
  * Gets the commit ID that the given `git_annotated_commit` refers to.
  *
  * @param commit the given annotated commit

src/annotated_commit.c

@@ -12,6 +12,7 @@
 #include "git2/refs.h"
 #include "git2/repository.h"
 #include "git2/annotated_commit.h"
+#include "git2/revparse.h"
 
 static int annotated_commit_init(
 	git_annotated_commit **out,
@@ -96,6 +97,33 @@ int git_annotated_commit_from_fetchhead(
 	return annotated_commit_init(out, repo, id, branch_name, remote_url);
 }
 
+int git_annotated_commit_from_revspec(
+	git_annotated_commit **out,
+	git_repository *repo,
+	const char *revspec)
+{
+	git_object *obj, *commit;
+	int error;
+
+	assert(out && repo && revspec);
+
+	if ((error = git_revparse_single(&obj, repo, revspec)) < 0)
+		return error;
+
+	if ((error = git_object_peel(&commit, obj, GIT_OBJ_COMMIT))) {
+		git_object_free(obj);
+		return error;
+	}
+
+	error = annotated_commit_init(out, repo, git_object_id(commit), revspec, NULL);
+
+	git_object_free(obj);
+	git_object_free(commit);
+
+	return error;
+}
+
+
 const git_oid *git_annotated_commit_id(
 	const git_annotated_commit *annotated_commit)
 {