Don't print env-supplied secrets in flag usage

[locker95]

--repo, --credential, and --askpass-url take their defaults from the environment. pflag then prints that default in --help and when flag parsing fails, so a token in GITSYNC_REPO or GITSYNC_CREDENTIAL lands in the sidecar log.

The startup line already redacts these. This only changes the text pflag shows for the default, and only when the env var is actually set. An empty --credential still prints []. The value that gets parsed is unchanged.

Fixes #1001

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