p0004: don't error out if test repo is too small

p0004: don't error out if test repo is too small

Repositories with less than 4000 entries are always handled using a
single thread, causing test-lazy-init-name-hash --multi to error out.
Don't abort the whole test script in that case, but simply skip the
multi-threaded performance check.  We can still use it to compare the
single-threaded speed of different versions in that case.

Signed-off-by: Rene Scharfe <redacted>
Acked-by: Jeff Hostetler <redacted>
Signed-off-by: Junio C Hamano <redacted>