plaintextstore: do not write creds if nothing has changed

Do not needlessly serialise the credential file again if nothing has
changed from an existing credential found.

This change also updates derived implementations (DPAPI and GPG).

Author: mjcheetham

src/shared/Core/PlaintextCredentialStore.cs

@@ -33,6 +33,16 @@ public void AddOrUpdate(string service, string account, string secret)
             // Ensure the store root exists and permissions are set
             EnsureStoreRoot();
 
+            FileCredential existingCredential = Enumerate(service, account).FirstOrDefault();
+
+            // No need to update existing credential if nothing has changed
+            if (existingCredential != null &&
+                StringComparer.Ordinal.Equals(account, existingCredential.Account) &&
+                StringComparer.Ordinal.Equals(secret, existingCredential.Password))
+            {
+                return;
+            }
+
             string serviceSlug = CreateServiceSlug(service);
             string servicePath = Path.Combine(StoreRoot, serviceSlug);