Fix CredentialHelper::add_command

Fixes #1136

Author: aibaars

src/cred.rs

@@ -299,13 +299,19 @@ impl CredentialHelper {
 
         if cmd.starts_with('!') {
             self.commands.push(cmd[1..].to_string());
-        } else if cmd.contains("/") || cmd.contains("\\") {
+        } else if is_absolute_path(cmd) {
             self.commands.push(cmd.to_string());
         } else {
             self.commands.push(format!("git credential-{}", cmd));
         }
     }
 
+    fn is_absolute_path(path: &str) -> bool {
+        path.starts_with('/')
+            || path.starts_with('\\')
+            || cfg!(windows) && path.chars().nth(1).is_some_and(|x| x == ':')
+    }
+
     fn exact_key(&self, name: &str) -> String {
         format!("credential.{}.{}", self.url, name)
     }