recognise GitLab hosts by WWW-Authenticate header (#1428)

Tested with https://salsa.debian.org/

Author: ldennington

src/shared/GitLab/GitLabHostProvider.cs

@@ -4,6 +4,7 @@
 using GitCredentialManager;
 using GitCredentialManager.Authentication.OAuth;
 using System.Net.Http.Headers;
+using System.Linq;
 
 namespace GitLab
 {
@@ -69,6 +70,11 @@ public override bool IsSupported(InputArguments input)
                 return true;
             }
 
+            if (input.WwwAuth.Any(x => x.Contains("realm=\"GitLab\"")))
+            {
+                return true;
+            }
+
             return false;
         }