Add gitSync optional env description

[1yangsh]

When fetching a remote git repository to a local directory,
optional command to be executed after syncing a new hash of the remote repository as occasion demands.

Added descriptions of several environmental variables for script execution.

GIT_SYNC_PERMISSION

• Change permissions on the checked-out files to the specified mode.

GIT_SYNC_EXECHOOK_BACKOFF (default: 3s)

• The time to wait before retrying a failed --exechook-command.

GIT_SYNC_EXECHOOK_COMMAND (script to be executed)

• An optional command can be executed after syncing a new hash of the remote repository.

GIT_SYNC_EXECHOOK_TIMEOUT (default: 30s)

• The timeout for the --exechook-command.

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^ Add meaningful description above

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[jedcunningham]

Thanks @1yangsh! Congrats on your first commit 🎉