Fix adding variable to config file with no trailing newline

This can occur after a manual modification of a config file.

src/config_file.c

@@ -1182,6 +1182,10 @@ static int config_write(diskfile_backend *cfg, const char *key, const regex_t *p
 				goto rewrite_fail;
 			}
 
+			/* If we are here, there is at least a section line */
+			if (*(cfg->reader.buffer.ptr + cfg->reader.buffer.size - 1) != '\n')
+				git_filebuf_write(&file, "\n", 1);
+
 			git_filebuf_printf(&file, "\t%s = %s\n", name, value);
 		}
 	}

tests-clar/config/write.c

@@ -3,11 +3,13 @@
 void test_config_write__initialize(void)
 {
 	cl_fixture_sandbox("config/config9");
+	cl_fixture_sandbox("config/config17");
 }
 
 void test_config_write__cleanup(void)
 {
 	cl_fixture_cleanup("config9");
+	cl_fixture_cleanup("config17");
 }
 
 void test_config_write__replace_value(void)
@@ -136,3 +138,19 @@ void test_config_write__escape_value(void)
 	cl_assert_equal_s(str, "this \"has\" quotes and \t");
 	git_config_free(cfg);
 }
+
+void test_config_write__add_value_in_file_with_no_clrf_at_the_end(void)
+{
+	git_config *cfg;
+	int i;
+
+	cl_git_pass(git_config_open_ondisk(&cfg, "config17"));
+	cl_git_pass(git_config_set_int32(cfg, "core.newline", 7));
+	git_config_free(cfg);
+
+	cl_git_pass(git_config_open_ondisk(&cfg, "config17"));
+	cl_git_pass(git_config_get_int32(&i, cfg, "core.newline"));
+	cl_assert_equal_i(7, i);
+
+	git_config_free(cfg);
+}

tests-clar/resources/config/config17

+[core]
+	dummy2 = 7
+	global = 17
\ No newline at end of file