github: support multiple accounts with explicit prompt

Check for multiple existing accounts, and if there is more than one
account with credentials in the store AND no user name hint provided in
the remote URL, then show the new account selection prompt.

Author: mjcheetham

src/shared/GitHub/GitHubHostProvider.cs

@@ -126,18 +126,53 @@ private static string GetServiceName(Uri baseUri)
 
         public async Task<ICredential> GetCredentialAsync(InputArguments input)
         {
-            // Try and locate an existing credential in the OS credential store
             string service = GetServiceName(input);
-            _context.Trace.WriteLine($"Looking for existing credential in store with service={service} account={input.UserName}...");
+            Uri remoteUri = input.GetRemoteUri();
+
+            // If we have a specific username then we can try and find an existing credential for that account.
+            // If not, we should check what accounts are available in the store and prompt the user if there
+            // are multiple options.
+            string userName = null;
+            bool addAccount = false;
+            if (string.IsNullOrWhiteSpace(input.UserName))
+            {
+                IList<string> accounts = _context.CredentialStore.GetAccounts(service);
+                _context.Trace.WriteLine($"Found {accounts.Count} accounts in the store for service={service}.");
+
+                switch (accounts.Count)
+                {
+                    case 1:
+                        userName = accounts[0];
+                        break;
+
+                    case > 1:
+                        userName = await _gitHubAuth.SelectAccountAsync(remoteUri, accounts);
+                        addAccount = userName is null;
+                        break;
+                }
+            }
+
+            // Always try and locate an existing credential in the OS credential store
+            // unless we're being told to explicitly add a new account. If the account lookup
+            // failed above we should still try to lookup an existing credential.
+            ICredential credential = null;
+            if (addAccount)
+            {
+                _context.Trace.WriteLine("Adding a new account!");
+            }
+            else
+            {
+                _context.Trace.WriteLine($"Looking for existing credential in store with service={service} account={userName}...");
+                credential = _context.CredentialStore.Get(service, userName);
+            }
 
-            ICredential credential = _context.CredentialStore.Get(service, input.UserName);
             if (credential == null)
             {
                 _context.Trace.WriteLine("No existing credentials found.");
 
                 // No existing credential was found, create a new one
                 _context.Trace.WriteLine("Creating new credential...");
-                credential = await GenerateCredentialAsync(input.GetRemoteUri(), input.UserName);
+                credential = await GenerateCredentialAsync(remoteUri, input.UserName);
                 _context.Trace.WriteLine("Credential created.");
             }
             else