Fix loading customCommands from per-repo config file

Any newly loaded custom command coming from the per-repo config file should add
to the global ones (or override an existing one in the global one), rather than
replace all global ones.

We can achieve this by simply prepending the newly loaded commands to the
existing ones. We don't have to take care of removing duplicate key assignments;
it is already possible to add two custom commands with the same key to the
global config file, the first one wins.

pkg/config/app_config.go

@@ -196,10 +196,14 @@ func loadUserConfig(configFiles []*ConfigFile, base *UserConfig) (*UserConfig, e
 			return nil, err
 		}
 
+		existingCustomCommands := base.CustomCommands
+
 		if err := yaml.Unmarshal(content, base); err != nil {
 			return nil, fmt.Errorf("The config at `%s` couldn't be parsed, please inspect it before opening up an issue.\n%w", path, err)
 		}
 
+		base.CustomCommands = append(base.CustomCommands, existingCustomCommands...)
+
 		if err := base.Validate(); err != nil {
 			return nil, fmt.Errorf("The config at `%s` has a validation error.\n%w", path, err)
 		}

pkg/integration/tests/config/custom_commands_in_per_repo_config.go

@@ -49,10 +49,7 @@ customCommands:
 		t.Views().Status().Content(Contains("other → master"))
 
 		t.GlobalPress("X")
-		/* EXPECTED:
 		t.FileSystem().FileContent("../other/file.txt", Equals("global X"))
-		ACTUAL: */
-		t.FileSystem().PathNotPresent("../other/file.txt")
 
 		t.GlobalPress("Y")
 		t.FileSystem().FileContent("../other/file.txt", Equals("local Y"))