auth.texi (Help for developers): Update docs to explain that the :save-function will only run the first time.
auth-source.el (auth-source-format-prompt): Always convert the value to a string to avoid evaluating non-string arguments.
(auth-source-netrc-create): Offer default properly, not as initial content in `read-string'.
(auth-source-netrc-saver): Use a cache keyed by file name and MD5 hash of line to determine if we've been run before. If so, don't run again, but print a trivial message to indicate the cache was hit instead.
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