contacts: validate hunk length earlier

contacts: validate hunk length earlier

Rather than calling get_blame() with a zero-length hunk only to have it
rejected immediately, perform hunk-length validation earlier in order to
avoid calling get_blame() unnecessarily.

This is a preparatory step to simplify later patches which reduce the
number of git-blame invocations by collecting together all lines to
blame within a single file at a particular revision. By validating the
blame range early, the subsequent patch can more easily avoid adding
empty ranges at collection time.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>