contacts: gather all blame sources prior to invoking git-blame

contacts: gather all blame sources prior to invoking git-blame

git-contacts invokes git-blame immediately upon encountering a patch
hunk. No attempt is made to consolidate invocations for multiple hunks
referencing the same file at the same revision. This can become
expensive quickly.

Any effort to reduce the number of times git-blame is run will need to
to know in advance which line ranges to blame per file per revision.
Make this information available by collecting all sources as a distinct
step from invoking git-blame.  A subsequent patch will utilize the
information to optimize git-blame invocations.

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