use compare API to deepen by exact divergence instead of fixed 1000

entry

@@ -146647,15 +146647,33 @@ async function checkoutPrBranch(pullNumber, params) {
   const baseBranch = pr.data.base.ref;
   const headBranch = pr.data.head.ref;
   const localBranch = `pr-${pullNumber}`;
+  const isShallow = $("git", ["rev-parse", "--is-shallow-repository"], { log: false }).trim() === "true";
+  let deepenArgs = [];
+  if (isShallow) {
+    let depth = 1e3;
+    try {
+      const comparison = await octokit.rest.repos.compareCommits({
+        owner,
+        repo: name,
+        base: baseBranch,
+        head: `pull/${pullNumber}/head`
+      });
+      depth = comparison.data.behind_by + 10;
+      log.debug(
+        `\xBB PR is ${comparison.data.behind_by} commits behind ${baseBranch}, deepening by ${depth}`
+      );
+    } catch {
+      log.debug(`\xBB compare API failed, falling back to --deepen=${depth}`);
+    }
+    deepenArgs = [`--deepen=${depth}`];
+  }
   const currentSha = $("git", ["rev-parse", "HEAD"], { log: false }).trim();
   const alreadyOnBranch = currentSha === pr.data.head.sha;
   if (alreadyOnBranch) {
     log.debug(`already on PR branch ${localBranch}, skipping checkout`);
   } else {
     log.debug(`\xBB fetching base branch (${baseBranch})...`);
-    const isShallow = $("git", ["rev-parse", "--is-shallow-repository"], { log: false }).trim();
-    const baseFetchArgs = isShallow === "true" ? ["--deepen=1000", "--no-tags", "origin", baseBranch] : ["--no-tags", "origin", baseBranch];
-    $git("fetch", baseFetchArgs, {
+    $git("fetch", [...deepenArgs, "--no-tags", "origin", baseBranch], {
       token: gitToken,
       restricted: shell !== "enabled"
     });
@@ -146670,9 +146688,7 @@ async function checkoutPrBranch(pullNumber, params) {
   }
   if (alreadyOnBranch) {
     log.debug(`\xBB fetching base branch (${baseBranch})...`);
-    const isShallow = $("git", ["rev-parse", "--is-shallow-repository"], { log: false }).trim();
-    const baseFetchArgs = isShallow === "true" ? ["--deepen=1000", "--no-tags", "origin", baseBranch] : ["--no-tags", "origin", baseBranch];
-    $git("fetch", baseFetchArgs, {
+    $git("fetch", [...deepenArgs, "--no-tags", "origin", baseBranch], {
       token: gitToken,
       restricted: shell !== "enabled"
     });