Usage: repo init -u https://github.com/LineageOS/mirror --mirror

Once the mirror is synced, you can then run repo init -u /path/to/mirror/LineageOS/android.git -b $BRANCHNAME and sync normally.

If you want to sync the source quickly but want it to be up-to-date without syncing the mirror every time, then run repo init -u http://www.github.com/LineageOS/android -b $BRANCHNAME --reference=/path/to/mirror/. This will init the new repo and fetch all the (available) data from the mirror, but will fallback to GitHub if something is missing in the mirror.

To update the mirror, use the mirror-regen.py script.
Please make sure you set the environment variables before using the script:

GHUSER contains a valid GitHub Username and
GHTOKEN contains a matching GitHub Personal Access Token

To set these environment variables, run these commands in your terminal window:

export GHUSER="<Your Username>"
export GHTOKEN="<Your Token>"

(You can obtain a GitHub Personal Access Token here)

WARNING: Please make sure no repositories have been removed before pushing a manifest change to Gerrit. A poor network connection could result in an incomplete manifest.