index: return a unique error code on dirty index

When the index is dirty, return GIT_EINDEXDIRTY so that consumers can
identify the exact problem programatically.

include/git2/errors.h

@@ -55,6 +55,7 @@ typedef enum {
 	GIT_ITEROVER        = -31,	/**< Signals end of iteration with iterator */
 	GIT_RETRY           = -32,	/**< Internal only */
 	GIT_EMISMATCH       = -33,	/**< Hashsum mismatch in object */
+	GIT_EINDEXDIRTY     = -34,	/**< Unsaved changes in the index would be overwritten */
 } git_error_code;
 
 /**

src/index.c

@@ -685,7 +685,7 @@ int git_index_read_safely(git_index *index)
 	if (index->dirty) {
 		giterr_set(GITERR_INDEX,
 			"the index has unsaved changes that would be overwritten by this operation");
-		return -1;
+		return GIT_EINDEXDIRTY;
 	}
 
 	return git_index_read(index, false);

tests/index/tests.c

@@ -384,6 +384,28 @@ void test_index_tests__dirty_and_clean(void)
 	git_repository_free(repo);
 }
 
+void test_index_tests__dirty_fails_with_error(void)
+{
+	git_repository *repo;
+	git_index *index;
+	git_index_entry entry = {{0}};
+
+	/* Index is not dirty after opening */
+	repo = cl_git_sandbox_init("testrepo");
+	cl_git_pass(git_repository_index(&index, repo));
+
+	/* Index is dirty after adding an entry */
+	entry.mode = GIT_FILEMODE_BLOB;
+	entry.path = "test.txt";
+	cl_git_pass(git_index_add_frombuffer(index, &entry, "Hi.\n", 4));
+	cl_assert(git_index_is_dirty(index));
+
+	cl_git_fail_with(GIT_EINDEXDIRTY, git_checkout_head(repo, NULL));
+
+	git_index_free(index);
+	cl_git_sandbox_cleanup();
+}
+
 void test_index_tests__add_frombuffer_reset_entry(void)
 {
 	git_index *index;