feat: diff on release flow

Also, don't commit unstaged files.

pkg/git/diff.go

@@ -0,0 +1,42 @@
+package git
+
+import (
+	"fmt"
+	"os/exec"
+
+	"github.com/sirupsen/logrus"
+)
+
+// getGitDiffArgs builds the `git diff` invocation args. It removes the usage
+// of a pager and ensures that colours are specified even when Git might detect
+// otherwise.
+func getGitDiffArgs(repoPath string) []string {
+	return []string{
+		"-C",
+		repoPath,
+		"--no-pager",
+		"-c",
+		"color.diff=always",
+		"diff",
+	}
+}
+
+// DiffUnstaged shows a `git diff`. Due to limitations in the underlying go-git
+// library, this implementation requires the /usr/bin/git binary. It gracefully
+// skips if it cannot find the command on the system.
+func DiffUnstaged(path string) error {
+	if _, err := exec.LookPath("git"); err != nil {
+		logrus.Warnf("unable to locate git command, cannot output diff")
+		return nil
+	}
+
+	gitDiffArgs := getGitDiffArgs(path)
+	diff, err := exec.Command("git", gitDiffArgs...).Output()
+	if err != nil {
+		return nil
+	}
+
+	fmt.Print(string(diff))
+
+	return nil
+}