Harden internal git profiles against non-standard diff configs

[svarlamov]

Summary

• introduce explicit internal git execution profiles (General, PatchParse, NumstatParse, RawDiffParse) and route parse-sensitive callsites through them
• remove implicit command-name-based diff rewriting and replace it with explicit profile application at callsites
• harden patch parsing for +++ headers to handle no-prefix/custom-prefix/mnemonic-style paths
• add hostile diff.* config integration coverage for simple additions, diff, stats, and status
• add centralized-spawn guard test to prevent direct internal git spawns outside approved helpers

Hardening fixes added after review

• profile conflict stripping now only rewrites option tokens after the git subcommand and before --, so pathspec tokens are never dropped
• NumstatParse now strips short rename/copy variants (-M*, -C*) so --no-renames cannot be overridden later in argv
• continue_session patch-producing git show now uses PatchParse profile for stable output under hostile diff.* config

Validation

• cargo test --lib numstat_profile_strips_short_rename_and_copy_flags
• cargo test --lib profile_rewrite_does_not_strip_pathspec_tokens_after_double_dash
• cargo test --tests --no-run
• cargo test --test simple_additions test_simple_additions_with_base_commit_and_custom_diff_config -- --nocapture
• cargo test --test diff test_diff_parsing_is_stable_under_hostile_diff_config -- --nocapture
• cargo test --test stats test_stats_cli_range_with_hostile_diff_config -- --nocapture
• cargo test --test status_ignore test_status_numstat_is_stable_under_hostile_diff_config -- --nocapture
• cargo test --test internal_spawn_safety direct_git_command_spawns_are_centralized -- --nocapture
• cargo test --test continue_session -- --nocapture
• cargo fmt -- --check

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[svarlamov]

Addressed Devin review in eb7fcc3c:

• replaced PatchParse --default-prefix with portable canonical flags --src-prefix=a/ and --dst-prefix=b/ (works on older Git versions)
• fixed conflict stripping for split-arg prefix forms (--src-prefix X, --dst-prefix X) so both flag and value are removed before canonical profile flags are applied
• added/updated unit coverage for both behaviors

Re-running CI on latest head now.