git: fix crash when reading stderr from non-redirected processes

ProcessManager.CreateProcess sets RedirectStandardError=false for all
processes to avoid TRACE2 deadlocks. However, GetRemotes and
CreateGitException unconditionally read StandardError, which throws
InvalidOperationException when stderr is not redirected.

Fix GetRemotes by explicitly redirecting stderr before starting the
process, since it needs to check for 'not a git repository' errors.
Guard CreateGitException defensively, as it is called from various
contexts where stderr may or may not be redirected.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

src/shared/Core/Git.cs

@@ -174,6 +174,8 @@ public IEnumerable<GitRemote> GetRemotes()
         {
             using (var git = CreateProcess("remote -v show"))
             {
+                // Redirect stderr so we can check for 'not a git repository' errors
+                git.StartInfo.RedirectStandardError = true;
                 git.Start(Trace2ProcessClass.Git);
                 // To avoid deadlocks, always read the output stream first and then wait
                 // TODO: don't read in all the data at once; stream it
@@ -274,7 +276,9 @@ public async Task<IDictionary<string, string>> InvokeHelperAsync(string args, ID
 
         public static GitException CreateGitException(ChildProcess git, string message, ITrace2 trace2 = null)
         {
-            var gitMessage = git.StandardError.ReadToEnd();
+            var gitMessage = git.StartInfo.RedirectStandardError
+                ? git.StandardError.ReadToEnd()
+                : null;
 
             if (trace2 != null)
                 throw new Trace2GitException(trace2, message, git.ExitCode, gitMessage);