fix: add an empty line between commit summary and body

[XYenon]

Description

This PR fixes the Git commit message format generated when using the --commit-changes option.

According to Git best practices and standards, there should be an empty line between the commit summary (subject line) and the commit body. Previously, nvfetcher used a single newline character \n, which caused the body to follow the summary immediately. This change introduces a second newline character to ensure a proper empty line.

Comparison

Before:

Update
package-a: 1.0.0 -> 1.1.0
package-b: 2.0.0 -> 2.1.0

After:

Update

package-a: 1.0.0 -> 1.1.0
package-b: 2.0.0 -> 2.1.0

Affected Scope

Only affects the automatic commit message format when the --commit-changes flag is used.

[berberman]

Thank you!