blame: consult diff process for no-hunk detection

blame: consult diff process for no-hunk detection

When a diff process is configured via diff.<driver>.process,
consult it during blame's per-commit diffing.  If the process
returns no hunks for a commit's changes to a file, treat the
commit as having no changes, causing blame to attribute lines
to earlier commits.

Introduce xdi_diff_process(), a process-aware xdi_diff() that
consults the process, runs xdiff on the tool's hunks or on the
builtin algorithm when it does not apply, frees the hunks, and
reports DIFF_PROCESS_EQUIVALENT (without running xdiff) so the caller
can drop or skip the change.  It is the shared consult-then-diff path
for consumers that work on raw hunks: blame's pass_blame_to_parent()
uses it here, and git log -L reuses it later.  builtin_diff() keeps
consulting the process directly, because it tests for equivalence
early, before its funcname-pattern and word-diff setup, so a
reformat-only file short-circuits without that work.

Blame's -w option is not communicated to the process and it could not
honor it, so blame must fall back to the builtin diff there.  Because
blame keeps its whitespace flags in sb->xdl_opts rather than diffopt,
the process bypass keys off xpp (the flags the diff actually runs
with), which covers blame without a guard of its own.

The subprocess is long-running (one startup cost amortized across the
blame traversal), but each commit in the file's history incurs a
round-trip to the tool.

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