Merge unreleased applies all changes to all projects

[TheSpyder]

Description
"merge unreleased" (#518) does not support projects correctly.

Reproduction Steps
Steps to reproduce the behavior:

1. Set up changie with projects configured
2. Add some unreleased changes specific to each project
3. Run changie merge -u "unreleased"

What happened
Every changelog has all unreleased changes.

Expected behavior
Only changes specific to a project should be added as unreleased changes in that project's changelog.

Additional context
This feature is very handy for monorepos - I was about to request it before I realised it was already an option - because not all projects in the monorepo have unreleased changes at the same time. I want to verify in CI that unreleased changie files are valid, and changie batch fails on projects without changes.

Perhaps broken during #557, when merge switched from per-project to all projects.

[TheSpyder]

Is it as simple as adding the project to the fake batch? That suggests this was broken before merge switched to all projects.

changie/cmd/merge.go

Lines 143 to 148 in 6f8dc52

	// create a fake batch to write the changes
	b := &Batch{
	config: cfg,
	writer: writer,
	TemplateCache: m.TemplateCache,
	}

[TheSpyder]

Right idea, wrong code 😅 It's the core.GetChanges() call.

git diff
diff --git a/cmd/merge.go b/cmd/merge.go
index 95907d0..9c99447 100644
--- a/cmd/merge.go
+++ b/cmd/merge.go
@@ -127,7 +127,7 @@ func (m *Merge) mergeProject(cfg *core.Config, project, changelogPath string) er
                        nil,
                        m.ReadDir,
                        m.ReadFile,
-                       "",
+                       project,
                )
                if unrelErr != nil {
                        return unrelErr