First implementation of authentication handling

README.md

@@ -63,3 +63,20 @@ needed. Optional text can be supplied to Git with `#:input`.
 The result remains two values: Git's exit code and the ordered
 `(source line)` output items.
 
+## Authentication retry
+
+Authentication failures are detected centrally after `run-git`, before
+command-specific result processing. `current-git-authentication-handler`
+contains a callback that receives the command, processed arguments and an
+`exn:fail:git-auth` value. If the callback handles authentication and returns a
+true value, git-cli retries the original command once. The default callback
+returns `#f`, so existing error behavior is preserved until an authentication
+handler is configured.
+
+```racket
+(current-git-authentication-handler
+ (λ (cmd args e)
+   ;; Perform credential handling here.
+   #t))
+```
+