line-log: consult diff process for range tracking

line-log: consult diff process for range tracking

git log -L tracks line ranges by diffing each commit against its
parent in collect_diff().  This pass used the builtin diff while the
displayed diff (builtin_diff()) consults a configured
diff.<driver>.process, so the two could disagree: a reformat-only
commit selected by builtin tracking was then rendered with an empty
diff because the tool reported the files equivalent.

Consult the process in collect_diff() too, mirroring the blame
integration.  When the tool reports the files equivalent, collect no
ranges; the tracked range then maps across unchanged and the commit
drops out of the log, matching what is displayed.  Like the summary
formats, the tracking pass diffs raw content, so the tool is consulted
on the raw blobs here.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>