branch: constness fixes

include/git2/branch.h

@@ -238,7 +238,7 @@ GIT_EXTERN(int) git_branch_upstream_name(
  * error code otherwise.
  */
 GIT_EXTERN(int) git_branch_is_head(
-	git_reference *branch);
+	const git_reference *branch);
 
 /**
  * Return the name of remote that the remote tracking branch belongs to.

include/git2/refs.h

@@ -586,7 +586,7 @@ GIT_EXTERN(int) git_reference_ensure_log(git_repository *repo, const char *refna
  * @return 1 when the reference lives in the refs/heads
  * namespace; 0 otherwise.
  */
-GIT_EXTERN(int) git_reference_is_branch(git_reference *ref);
+GIT_EXTERN(int) git_reference_is_branch(const git_reference *ref);
 
 /**
  * Check if a reference is a remote tracking branch

src/branch.c

@@ -588,7 +588,7 @@ on_error:
 }
 
 int git_branch_is_head(
-		git_reference *branch)
+		const git_reference *branch)
 {
 	git_reference *head;
 	bool is_same = false;

src/refs.c

@@ -1137,7 +1137,7 @@ int git_reference__is_branch(const char *ref_name)
 	return git__prefixcmp(ref_name, GIT_REFS_HEADS_DIR) == 0;
 }
 
-int git_reference_is_branch(git_reference *ref)
+int git_reference_is_branch(const git_reference *ref)
 {
 	assert(ref);
 	return git_reference__is_branch(ref->name);