line-log: route -L output through the standard diff pipeline

`git log -L` has always bypassed the standard diff pipeline.
`dump_diff_hacky()` in line-log.c hand-rolls its own diff headers and
hunk output, which means most diff formatting options are silently
ignored.  A NEEDSWORK comment has acknowledged this since the feature
was introduced:

    /*
     * NEEDSWORK: manually building a diff here is not the Right
     * Thing(tm).  log -L should be built into the diff pipeline.
     */

Remove `dump_diff_hacky()` and its helpers and route -L output through
`builtin_diff()` / `fn_out_consume()`, the same path used by `git diff`
and `git log -p`.  The mechanism is a pair of callback wrappers that sit
between `xdi_diff_outf()` and `fn_out_consume()`, filtering xdiff's
output to only the tracked line ranges.  To ensure xdiff emits all lines
within each range as context, the context length is inflated to span the
largest range.

Wire up the `-L` implies `--patch` default in revision setup rather
than forcing it at output time, so `line_log_print()` is just
`diffcore_std()` + `diff_flush()` with no format save/restore.
Rename detection is a no-op since pairs are already resolved during
the history walk in `queue_diffs()`, but running `diffcore_std()`
means `-S`/`-G` (pickaxe), `--orderfile`, and `--diff-filter` now
work with `-L`, and `diff_resolve_rename_copy()` sets pair statuses
correctly without manual assignment.

Switch `diff_filepair_dup()` from `xmalloc` to `xcalloc` so that new
fields (including `line_ranges`) are zero-initialized by default.

As a result, diff formatting options that were previously silently
ignored (e.g. --word-diff, --no-prefix, -w, --color-moved) now work
with -L, and output gains `index` lines, `new file mode` headers, and
funcname context in `@@` headers.  This is a user-visible output change:
tools that parse -L output may need to handle the additional header
lines.

The context-length inflation means xdiff may process more output than
needed for very wide line ranges, but benchmarks on files up to 7800
lines show no measurable regression.

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

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