feat: Support short hex colors #RGB for compatibility with git 2.46

git 2.46 adds support for 12-bit hex colors, written `#RGB` with three
hex digits, in addition to the existing 24-bit `#RRGGBB` support with
six. Add support for this.

crates/anstyle-git/src/lib.rs

@@ -110,11 +110,16 @@ fn parse_color(word: &str) -> Result<Option<anstyle::Color>, ()> {
         "cyan" => Some(anstyle::AnsiColor::Cyan.into()),
         "white" => Some(anstyle::AnsiColor::White.into()),
         _ => {
-            if word.starts_with('#') && word.len() == 7 {
+            if let Some(hex) = word.strip_prefix('#') {
+                let l = hex.len();
+                if l != 3 && l != 6 {
+                    return Err(());
+                }
+                let l = l / 3;
                 if let (Ok(r), Ok(g), Ok(b)) = (
-                    u8::from_str_radix(&word[1..3], 16),
-                    u8::from_str_radix(&word[3..5], 16),
-                    u8::from_str_radix(&word[5..7], 16),
+                    u8::from_str_radix(&hex[0..l], 16),
+                    u8::from_str_radix(&hex[l..(2 * l)], 16),
+                    u8::from_str_radix(&hex[(2 * l)..(3 * l)], 16),
                 ) {
                     Some(anstyle::Color::from((r, g, b)))
                 } else {