crendential-store: use timeout when locking file

crendential-store: use timeout when locking file

When holding the lock for rewriting the credential file, use a timeout
to avoid race conditions when the credentials file needs to be updated
in parallel.

An example would be doing `fetch --all` on a repository with several
remotes that need credentials, using parallel fetching.

The timeout can be configured using "credentialStore.lockTimeoutMS",
defaulting to 1 second.

Signed-off-by: Simão Afonso <simao.afonso@powertools-tech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>