Enable true colors if the terminfo RGB capability is set

This allows delta to work properly on Direct Color terminals like
xterm-direct and foot-direct.

This adds a dependency on the "terminfo" crate in order to read the
`RGB` capability. The crate is used by wezterm, among others.

Fixes #1931

Cargo.toml

@@ -54,6 +54,7 @@ syntect = "5.0.0"
 # sysinfo: no default features to disable the use of threads
 sysinfo = { version = "0.29.0", default-features = false, features = [] }
 terminal-colorsaurus = "0.4.1"
+terminfo = "0.9.0"
 unicode-segmentation = "1.10.1"
 unicode-width = "=0.1.12"
 xdg = "2.4.1"

src/options/set.rs

@@ -662,10 +662,14 @@ fn set_true_color(opt: &mut cli::Opt) {
 }
 
 fn is_truecolor_terminal(env: &DeltaEnv) -> bool {
-    env.colorterm
+    let rgb_cap = terminfo::Database::from_env()
+        .map(|db| db.raw("RGB").is_some())
+        .unwrap_or(false);
+    let colorterm = env.colorterm
         .as_ref()
         .map(|colorterm| colorterm == "truecolor" || colorterm == "24bit")
-        .unwrap_or(false)
+        .unwrap_or(false);
+    rgb_cap || colorterm
 }
 
 #[cfg(test)]