git: don't crash and burn when we're not in a Git repo

git.ts

···
 import $ from "@david/dax";
+import { existsSync } from "@std/fs/exists";
 import { join } from "@std/path";
 
 import { Notebook } from "./notebook.ts";
 
+export function hasGit(notebook: Notebook): boolean {
+  return existsSync(join(notebook.root, ".git"), { isDirectory: true });
+}
+
+export async function hasRemote(notebook: Notebook): Promise<boolean> {
+  if (!hasGit(notebook)) return false;
+  const remotes = await $`git remote`.cwd(notebook.root).text();
+  return remotes.trim() !== "";
+}
+
 export async function commit(notebook: Notebook, message: string) {
+  if (!hasGit(notebook)) return;
   const changed = await $`git status --porcelain`.cwd(notebook.root).text();
   if (changed.trim() === "") return;
   await $`git add .`.cwd(notebook.root);
···
 }
 
 export async function syncNotebook(notebook: Notebook, force: boolean = false) {
+  if (!hasGit(notebook)) return;
+
   const root = notebook.root;
   const status = await $`git status --porcelain`.cwd(root).text();
   if (status.trim() !== "") {
     await commit(notebook, "Checkpointing untracked changes");
   }
+
+  if (!await hasRemote(notebook)) return;
+
   const last = lastSyncTime(notebook);
   // FIXME(bmps): Make time configurable
   if (
···
   root: string,
   relpath: string,
 ): Promise<Temporal.Instant> {
+  const fullPath = join(root, relpath);
+  if (!existsSync(join(root, ".git"), { isDirectory: true })) {
+    const stat = Deno.statSync(fullPath);
+    return (stat.birthtime ?? stat.mtime ?? new Date()).toTemporalInstant();
+  }
   const out =
     (await $`git --no-pager log --reverse --date=iso-local --format="%aI" -- ${relpath}`
       .cwd(root).env({ "TZ": "UTC" }).text()).split("\n")[0];
+  if (!out) {
+    const stat = Deno.statSync(fullPath);
+    return (stat.birthtime ?? stat.mtime ?? new Date()).toTemporalInstant();
+  }
   return Temporal.Instant.from(out);
 }
 
···
   root: string,
   relpath: string,
 ): Promise<Temporal.Instant> {
+  const fullPath = join(root, relpath);
+  if (!existsSync(join(root, ".git"), { isDirectory: true })) {
+    const stat = Deno.statSync(fullPath);
+    return (stat.mtime ?? new Date()).toTemporalInstant();
+  }
   const out =
     (await $`git --no-pager log --date=iso-local --format="%aI" -- ${relpath}`
       .cwd(root).env({ "TZ": "UTC" }).text()).split("\n")[0];
+  if (!out) {
+    const stat = Deno.statSync(fullPath);
+    return (stat.mtime ?? new Date()).toTemporalInstant();
+  }
   return Temporal.Instant.from(out);
 }