fix: use git commit -F - to preserve newlines in commit messages

commitFileWithBody passed the message through JSON.stringify with
git commit -m, which escaped newlines to literal \n characters.
Switch to git commit -F - with stdin input to preserve actual newlines.

src/git.ts

@@ -18,7 +18,7 @@ export function commitFile(path: string, message: string, cwd = process.cwd()):
 export function commitFileWithBody(path: string, header: string, body: string, cwd = process.cwd()): void {
   execSync(`git add ${path}`, { cwd })
   const msg = `${header}\n\n${body}`
-  execSync(`git commit -m ${JSON.stringify(msg)}`, { cwd })
+  execSync(`git commit -F -`, { cwd, input: msg })
 }
 
 export function branchExists(branch: string, cwd = process.cwd()): boolean {