fix(util/git): log date as string

As a follow-up to changes in 7ebba26
which accidentally changed this behaviour, as we're now passing in the
`DateTime`.

Author: jamietanna

lib/util/git/index.ts

@@ -626,7 +626,11 @@ export async function checkoutBranch(
     // v8 ignore else -- TODO: add test #40625
     if (latestCommitDate) {
       logger.debug(
-        { branchName, latestCommitDate, sha: config.currentBranchSha },
+        {
+          branchName,
+          latestCommitDate: latestCommitDate.toISO(),
+          sha: config.currentBranchSha,
+        },
         'latest commit',
       );
     }