fast-export: --signed-commits is experimental

As the design of signature handling is still being discussed, it is
likely that the data stream produced by the code in Git 2.50 would
have to be changed in such a way that is not backward compatible.

Mark the feature as experimental and discourge its use for now.

Also flip the default on the generation side to "strip"; users of
existing versions would not have passed --signed-commits=strip and
will be broken by this change if the default is made to abort, and
will be encouraged by the error message to produce data stream with
future breakage guarantees by passing --signed-commits option.

As we tone down the default behaviour, we no longer need the
FAST_EXPORT_SIGNED_COMMITS_NOABORT environment variable, which was
not discoverable enough.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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