devops: speedup initial browser checkout (#4352)

Instead of checking out the whole repository, we now do a shallow
clone.

We then gradually "unshallow" the clone, looking for the `BASE_REVISION`.

This should fix experimental mac-11 builder.

Author: aslushnikov

browser_patches/prepare_checkout.sh

@@ -71,7 +71,7 @@ fi
 # if there's no checkout folder - checkout one.
 if ! [[ -d $CHECKOUT_PATH ]]; then
   echo "-- $FRIENDLY_CHECKOUT_PATH is missing - checking out.."
-  git clone --single-branch --branch $BASE_BRANCH $REMOTE_URL $CHECKOUT_PATH
+  git clone --single-branch --depth 1 --branch $BASE_BRANCH $REMOTE_URL $CHECKOUT_PATH
 else
   echo "-- checking $FRIENDLY_CHECKOUT_PATH folder - OK"
 fi
@@ -102,11 +102,30 @@ else
   git remote add $REMOTE_BROWSER_UPSTREAM $REMOTE_URL
 fi
 
-# If not, fetch from REMOTE_BROWSER_UPSTREAM and check one more time.
-git fetch $REMOTE_BROWSER_UPSTREAM $BASE_BRANCH
-if ! git cat-file -e $BASE_REVISION^{commit}; then
-  echo "ERROR: $FRIENDLY_CHECKOUT_PATH/ does not include the BASE_REVISION (@$BASE_REVISION). Wrong revision number?"
-  exit 1
+# Check if our checkout contains BASE_REVISION.
+# If not, fetch from REMOTE_BROWSER_UPSTREAM and slowly fetch more and more commits
+# until we find $BASE_REVISION.
+# This technique allows us start with a shallow clone.
+if ! git cat-file -e $BASE_REVISION^{commit} 2>/dev/null; then
+  # Detach git head so that we can fetch into branch.
+  git checkout --detach >/dev/null 2>/dev/null
+
+  # Fetch 128 commits first, and then double the amount every iteration.
+  FETCH_DEPTH=128
+  SUCCESS="no"
+  while (( FETCH_DEPTH <= 8192 )); do
+    echo "Fetching ${FETCH_DEPTH} commits to find base revision..."
+    git fetch --depth "${FETCH_DEPTH}" $REMOTE_BROWSER_UPSTREAM $BASE_BRANCH
+    FETCH_DEPTH=$(( FETCH_DEPTH * 2 ));
+    if git cat-file -e $BASE_REVISION^{commit} >/dev/null; then
+      SUCCESS="yes"
+      break;
+    fi
+  done
+  if [[ "${SUCCESS}" == "no" ]]; then
+    echo "ERROR: $FRIENDLY_CHECKOUT_PATH/ does not include the BASE_REVISION (@$BASE_REVISION). Wrong revision number?"
+    exit 1
+  fi
 fi
 
 echo "-- checking $FRIENDLY_CHECKOUT_PATH repo has BASE_REVISION (@$BASE_REVISION) commit - OK"