git: consistently read from stdout before exit wait (#1136)

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.

This is important if Git ever writes out so much data to stdout that we
buffer the output, and then Git will not exit until it can finish
writing (and we'd be waiting for Git to exit).

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();