credstore: add no-op credential storage option

Add a null/no-op credential store option that, as the name suggests,
does nothing. This can be useful if the user wants to use another
credential helper, configured in-front of GCM via Git, to store
credentials.

Example config:

```ini
[credential]
	credentialStore = none
	helper = /bin/my-awesome-helper
	helper = /usr/local/bin/git-credential-manager
```

In this example, the `my-awesome-helper` will be consulted first to
retrieve existing credentials before GCM, and will be asked to store
any credentials generated by GCM.

docs/configuration.md

@@ -585,6 +585,7 @@ _(unset)_|Windows: `wincredman`, macOS: `keychain`, Linux: _(none)_|-
 `gpg`|Use GPG to store encrypted files that are compatible with the [pass][pass] (requires GPG and `pass` to initialize the store).|macOS, Linux
 `cache`|Git's built-in [credential cache][credential-cache].|macOS, Linux
 `plaintext`|Store credentials in plaintext files (**UNSECURE**). Customize the plaintext store location with [`credential.plaintextStorePath`][credential-plaintextstorepath].|Windows, macOS, Linux
+`none`|Do not store credentials via GCM.|Windows, macOS, Linux
 
 #### Example
 

docs/credstores.md

@@ -9,6 +9,7 @@ There are several options for storing credentials that GCM supports:
 - GPG/[`pass`][passwordstore] compatible files
 - Git's built-in [credential cache][credential-cache]
 - Plaintext files
+- Passthrough/no-op (no credential store)
 
 The default credential stores on macOS and Windows are the macOS Keychain and
 the Windows Credential Manager, respectively.
@@ -251,6 +252,31 @@ permissions on this directory such that no other users or applications can
 access files within. If possible, use a path that exists on an external volume
 that you take with you and use full-disk encryption.
 
+## Passthrough/no-op (no credential store)
+
+**Available on:** _Windows, macOS, Linux_
+
+**:warning: .**
+
+```batch
+SET GCM_CREDENTIAL_STORE="none"
+```
+
+or
+
+```shell
+git config --global credential.credentialStore none
+```
+
+This option disables the internal credential store. All operations to store or
+retrieve credentials will do nothing, and will return success. This is useful if
+you want to use a different credential store, chained in sequence via Git
+configuration, and don't want GCM to store credentials.
+
+Note that you'll want to ensure that another credential helper is placed before
+GCM in the `credential.helper` Git configuration or else you will be prompted to
+enter your credentials every time you interact with a remote repository.
+
 [access-windows-credential-manager]: https://support.microsoft.com/en-us/windows/accessing-credential-manager-1b5c916a-6a16-889f-8581-fc16e8165ac0
 [aws-cloudshell]: https://aws.amazon.com/cloudshell/
 [azure-cloudshell]: https://docs.microsoft.com/azure/cloud-shell/overview

docs/environment.md

@@ -716,6 +716,7 @@ _(unset)_|Windows: `wincredman`, macOS: `keychain`, Linux: _(none)_|-
 `gpg`|Use GPG to store encrypted files that are compatible with the [`pass` utility][passwordstore] (requires GPG and `pass` to initialize the store).|macOS, Linux
 `cache`|Git's built-in [credential cache][git-credential-cache].|Windows, macOS, Linux
 `plaintext`|Store credentials in plaintext files (**UNSECURE**). Customize the plaintext store location with [`GCM_PLAINTEXT_STORE_PATH`][gcm-plaintext-store-path].|Windows, macOS, Linux
+`none`|Do not store credentials via GCM.|Windows, macOS, Linux
 
 #### Windows
 

src/shared/Core/Constants.cs

@@ -38,6 +38,7 @@ public static class CredentialStoreNames
             public const string SecretService = "secretservice";
             public const string Plaintext = "plaintext";
             public const string Cache = "cache";
+            public const string None = "none";
         }
 
         public static class RegexPatterns

src/shared/Core/CredentialStore.cs

@@ -100,6 +100,10 @@ private void EnsureBackingStore()
                     _backingStore = new PlaintextCredentialStore(_context.FileSystem, plainStoreRoot, ns);
                     break;
 
+                case StoreNames.None:
+                    _backingStore = new NullCredentialStore();
+                    break;
+
                 default:
                     var sb = new StringBuilder();
                     sb.AppendLine(string.IsNullOrWhiteSpace(credStoreName)
@@ -168,6 +172,9 @@ private static void AppendAvailableStoreList(StringBuilder sb)
 
             sb.AppendFormat("  {1,-13} : store credentials in plain-text files (UNSECURE){0}",
                 Environment.NewLine, StoreNames.Plaintext);
+
+            sb.AppendFormat("  {1, -13} : disable internal credential storage{0}",
+                Environment.NewLine, StoreNames.None);
         }
 
         private void ValidateWindowsCredentialManager()

src/shared/Core/NullCredentialStore.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+
+namespace GitCredentialManager;
+
+/// <summary>
+/// Credential store that does nothing. This is useful when you want to disable internal credential storage
+/// and only use another helper configured in Git to store credentials.
+/// </summary>
+public class NullCredentialStore : ICredentialStore
+{
+    public IList<string> GetAccounts(string service) => Array.Empty<string>();
+
+    public ICredential Get(string service, string account) => null;
+
+    public void AddOrUpdate(string service, string account, string secret) { }
+
+    public bool Remove(string service, string account) => false;
+}