fast-import: tighten parsing of boolean command line options

fast-import: tighten parsing of boolean command line options

We parse options like "--max-pack-size=" using skip_prefix(), which
makes sense to get at the bytes after the "=". However, we also parse
"--quiet" and "--stats" with skip_prefix(), which allows things like
"--quiet-nonsense" to behave like "--quiet".

This was a mistaken conversion in 0f6927c229 (fast-import: put option
parsing code in separate functions, 2009-12-04). Let's tighten this to
an exact match, which was the original intent.

Signed-off-by: Jeff King <peff@peff.net>