[Git] Fix deadlock when running external authentication

Since all Git operations run on a dedicated thread, we can not
simply run async tasks in callbacks from libgit. Running
the external creds provider on a separate task fixes the deadlock.

Author: sevoku

main/src/addins/VersionControl/MonoDevelop.VersionControl.Git/MonoDevelop.VersionControl.Git/GitCredentials.cs

@@ -33,6 +33,7 @@
 using MonoDevelop.Components;
 using System.Linq;
 using Mono.Addins;
+using System.Threading.Tasks;
 
 namespace MonoDevelop.VersionControl.Git
 {
@@ -237,7 +238,7 @@ static GitCredentialsProviderResult GetCredentialsFromProvider (IGitCredentialsP
 			if (type != SupportedCredentialTypes.UsernamePassword)
 				return GitCredentialsProviderResult.NotFound;
 
-			var (result, credentials) = gitCredentialsProvider.TryGetCredentialsAsync (uri).Result;
+			var (result, credentials) = Task.Run (() => gitCredentialsProvider.TryGetCredentialsAsync (uri)).Result;
 
 			if (result == GitCredentialsProviderResult.Found) {
 				((UsernamePasswordCredentials)cred).Username = credentials.Username;