feat: less repetitive entries for for prerelease changelogs

Right now we always show all new releasable commits since the last
_stable_ release.

If a project publishes multiple pre-releases in a row, they all repeat
the same lines with a few new additions.

On the other hand, when we cut a stable release, we do want to show all
changes since the last stable release, as we can not expect users to
read the changelog of pre-releases.

As a compromise, the code now looks at the type of release that is being
created, and decides based on that if we will look at STABLE..HEAD
(stable release) or LATEST..HEAD (pre-release).

Close #33

releasepr.go

@@ -59,9 +59,8 @@ func NewReleasePullRequest(head, branch, version, changelogEntry string) (*Relea
 }
 
 type ReleaseOverrides struct {
-	Prefix string
-	Suffix string
-	// TODO: Doing the changelog for normal releases after previews requires to know about this while fetching the commits
+	Prefix          string
+	Suffix          string
 	NextVersionType NextVersionType
 }
 
@@ -92,6 +91,17 @@ func (n NextVersionType) String() string {
 	}
 }
 
+func (n NextVersionType) IsPrerelease() bool {
+	switch n {
+	case NextVersionTypeRC, NextVersionTypeBeta, NextVersionTypeAlpha:
+		return true
+	case NextVersionTypeUndefined, NextVersionTypeNormal:
+		return false
+	default:
+		return false
+	}
+}
+
 // Label is the string identifier of a pull/merge request label on the forge.
 type Label string