wincredmgr: only match cred entries with correct namespace

GCM by default creates entries in the Windows Credential Manager on
Windows, and prefixes the 'target name' of the entry with "git:".
This 'namespace' prefix is configurable, but is not often changed in
practice outside of tests.

Visual Studio, when adding GitHub accounts (either natively or by the
older GitHub extension for VS), it creates three credential entries:

1. GitHub for Visual Studio - https://github.com
2. git:https://github.com
3. https://github.com

Entry 1 is used by VS for it's own purposes. Entry 2 is created for the
benefit for GCM, so that we are 'primed'. It is unknown what entry 3 is
for at this time.

There is an error in our existing logic for enumerating credentials that
is also matching entry 3 as well as the expected entry 2.

Modify and fix the matching logic to ensure that the namespace prefix
matches, rather than just stripping it and matching (even if it doesn't
exist!).

Author: mjcheetham

src/shared/Core/Interop/Windows/WindowsCredentialManager.cs

@@ -274,11 +274,19 @@ private WindowsCredential CreateCredentialFromStructure(Win32Credential credenti
                 return false;
             }
 
-            // Trim the "LegacyGeneric" prefix Windows adds and any namespace we have been filtered with
+            // Trim the "LegacyGeneric" prefix Windows adds
             string targetName = credential.TargetName.TrimUntilIndexOf(TargetNameLegacyGenericPrefix);
+            
+            // Only match credentials with the namespace we have been configured with (if any)
             if (!string.IsNullOrWhiteSpace(_namespace))
             {
-                targetName = targetName.TrimUntilIndexOf($"{_namespace}:");
+                string nsPrefix = $"{_namespace}:";
+                if (!targetName.StartsWith(nsPrefix, StringComparison.Ordinal))
+                {
+                    return false;
+                }
+
+                targetName = targetName.Substring(nsPrefix.Length);
             }
 
             // If the target name matches the service name exactly then return 'match'