cmd/gorelease: migrated git status --porcelain to git diff

The expectation of this application is that it is to be utilized in release
pipelines. Due to this expectation, pipelines utilize volume mounting to
transfer artifacts (untracked files) across different jobs (specifically
referring to GITLAB here).

This is a problem because git status --porcelain checks for untracked files
and returns them as a list. This causes this pipeline to fail unexpectedly.

In testing, `git reset --hard` will not remove these, while a
`git clean -fdx` will correctly remove these files.

I would recommend that in a follow-up commit there is an update to the
available args for this cmd to control if untracked files are to be
included or not (eg: --untracked)

cmd/gorelease/gorelease.go

@@ -1507,7 +1507,7 @@ func shortRetractionRationale(rationales []string) (string, bool) {
 // changes.
 func hasGitUncommittedChanges(dir string) (bool, error) {
 	stdout := &bytes.Buffer{}
-	cmd := exec.Command("git", "status", "--porcelain")
+	cmd := exec.Command("git", "diff")
 	cmd.Dir = dir
 	cmd.Stdout = stdout
 	if err := cmd.Run(); err != nil {