Git fetcher: Do not consider a null revision (i.e. workdir) to be locked

[edolstra]

Motivation

The Git fetcher returns a rev of 0000... for dirty workdirs. This caused isLocked() to consider such inputs to be locked, which is incorrect.

(Probably the fetcher shouldn't return that 0000... rev in the first place, but we're doing that for backwards compatibility.)

Context

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