Let --show-themes accept --dark and --light together

[VXNCXNX]

What's broken

--show-themes --dark --light is documented and does not work.

The help text for show_themes in src/cli.rs says:

To control the themes shown, use --dark or --light, or both, on the command line together with this option.

But:

$ delta --show-themes --dark --light
--light and --dark cannot be used together.

The cause

validate_light_and_dark in src/options/set.rs runs before any subcommand dispatch, so it kills --show-themes along with everything else.

The intent is still in the tree: src/subcommands/show_themes.rs:64 has a || (dark && light) branch that is currently unreachable. It was written for exactly this.

The fix

Exempt --show-themes from the mutual-exclusion check, which re-arms that branch. Nothing in show_themes.rs changes.

Two things I checked

opt.light and opt.dark feed only theme::get_color_mode, where light wins if both are set. That resolves to a valid, fully populated config, and it is only used for the outer shell: the per-theme configs inside the loop are rebuilt from --features <theme>, so the outer colour mode does not affect what is rendered.

--show-syntax-themes makes no such promise, its help text never mentions the flags, so I did not extend the fix to it. --show-syntax-themes --dark --light still exits fatally, the same before and after.

Verification

With a HOME holding a themes gitconfig containing one dark and one light theme:

--dark           -> 1 theme
--light          -> 1 theme
--dark --light   -> 2 themes

And the guard still does its job for normal use:

$ delta --dark --light
--light and --dark cannot be used together.

cargo fmt --all --check, cargo clippy -- -D warnings, cargo build --release and cargo test (438 passed) are all clean, plus tests/test_deprecated_options.

The new integration test spawns the binary with its own HOME, because GitConfig::try_create returns None under cfg(test), so a unit test finds no themes at all. Reverting set.rs with the test kept makes it fail.

One note unrelated to this change: cargo clippy --all-targets -- -D warnings reports 4 needless_borrow errors in src/utils/process.rs. I confirmed those on a clean tree. CI's gate is cargo clippy -- -D warnings without --all-targets, which passes.

There is no issue for this one, I noticed it while reading the help text.