fix(git): preserve patch content when git log -p is used

[ousamabenyounes]

Problem

When agents use git log -p to inspect git history, RTK strips the patch/diff content, replacing it with [+N lines omitted] markers. The -p flag explicitly requests patch output, and this diff context is vital for diagnosing problems.

Root cause: with no format flag, RTK injects --pretty=format:…%b%n---END--- and then runs filter_log_output, which treats the diff hunks as commit body — keeping only 3 lines and discarding the rest behind [+N lines omitted].

Fix

When -p / --patch is detected, RTK skips its --pretty/limit/--no-merges defaults and the commit-block filter, printing git's raw output verbatim. Normal git log / --oneline behaviour is unchanged.

The filter-vs-passthrough decision is now a single pure function (finalize_log_output), and run_log builds the command once instead of duplicating the exec/error/print/timer block.

Testing — RED → GREEN

New unit tests over a real git log -p fixture (tests/fixtures/git_log_p_raw.txt).

GREEN — with the fix:

running 3 tests
test ...test_wants_patch_detects_explicit_flags ... ok
test ...test_finalize_preserves_patch_when_requested ... ok
test ...test_finalize_filters_when_no_patch_flag ... ok
test result: ok. 3 passed; 0 failed

RED — fix reverted (finalize_log_output always filters):

test ...test_finalize_preserves_patch_when_requested ... FAILED

assertion `left == right` failed: patch output must be byte-identical passthrough
  left: "commit a1ef0ef…\n  Author: …\n  Date:   …\n  [+17 lines omitted]"
 right: "commit a1ef0ef…\n…\ndiff --git a/Cargo.toml b/Cargo.toml\n@@ -1,6 +1,6 @@\n-version = \"0.42.1\"\n+version = \"0.42.2\"\n…"

test result: FAILED. 2 passed; 1 failed

The reverted code collapses the entire diff into [+17 lines omitted]; the fix preserves every hunk.

Full suite + lint:

cargo test --bin rtk   → 2070 passed; 0 failed; 7 ignored   (was 2067)
cargo fmt --all -- --check                 → clean
cargo clippy --all-targets -- -D warnings  → clean

Note: no token-savings test here — patch passthrough is intentionally a 0%-reduction path (the diff is the payload).

Closes #2275