git: consistently read from stdout before exit wait

We should always try and drain standard output before waiting for the
Git process to exit. We do this for all other calls that expect output,
but missed the `TryGet` method.

Author: mjcheetham

src/shared/Core/GitConfiguration.cs

@@ -199,6 +199,9 @@ public bool TryGet(GitConfigurationLevel level, GitConfigurationType type, strin
             using (Process git = _git.CreateProcess($"config --null {levelArg} {typeArg} {QuoteCmdArg(name)}"))
             {
                 git.Start();
+                // To avoid deadlocks, always read the output stream first and then wait
+                // TODO: don't read in all the data at once; stream it
+                string data = git.StandardOutput.ReadToEnd();
                 git.WaitForExit();
 
                 switch (git.ExitCode)
@@ -214,7 +217,6 @@ public bool TryGet(GitConfigurationLevel level, GitConfigurationType type, strin
                         return false;
                 }
 
-                string data = git.StandardOutput.ReadToEnd();
                 string[] entries = data.Split('\0');
                 if (entries.Length > 0)
                 {
@@ -301,7 +303,7 @@ public IEnumerable<string> GetAll(GitConfigurationLevel level, GitConfigurationT
             using (Process git = _git.CreateProcess(gitArgs))
             {
                 git.Start();
-
+                // To avoid deadlocks, always read the output stream first and then wait
                 // TODO: don't read in all the data at once; stream it
                 string data = git.StandardOutput.ReadToEnd();
                 git.WaitForExit();