gitconfig: avoid deadlock on large amounts of config data

As per the Remarks section in
https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.processstartinfo.redirectstandardoutput
we should first read the output until the end before waiting for the
`git config` process to finish, to avoid deadlocks.

This indeed helps in the situation of this developer whose config is so
large that `git config -l` produces 25 kilobyte worth of output.
Apparently, this fills up some buffer that seems to be around 8kB and
then `git config -l` waits for its output to be consumed, and before
this patch, the consumer (i.e. GCM Core) would wait for `git config -l`
to exit already.

Author: dscho

src/shared/Microsoft.Git.CredentialManager/GitConfiguration.cs

@@ -101,6 +101,9 @@ public void Enumerate(GitConfigurationEnumerationCallback cb)
             using (Process git = _git.CreateProcess($"config --null {level} --list"))
             {
                 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)
@@ -112,9 +115,6 @@ public void Enumerate(GitConfigurationEnumerationCallback cb)
                             $"Failed to enumerate all Git configuration entries. Exit code '{git.ExitCode}' (level={_filterLevel})");
                 }
 
-                // TODO: don't read in all the data at once; stream it
-                string data = git.StandardOutput.ReadToEnd();
-
                 IEnumerable<string> entries = data.Split('\0').Where(x => !string.IsNullOrWhiteSpace(x));
                 foreach (string entry in entries)
                 {
@@ -217,6 +217,9 @@ public IEnumerable<string> GetRegex(string nameRegex, string valueRegex)
             using (Process git = _git.CreateProcess($"config --null {level} --get-regex {nameRegex} {valueRegex}"))
             {
                 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)
@@ -229,9 +232,6 @@ public IEnumerable<string> GetRegex(string nameRegex, string valueRegex)
                             $"Failed to get Git configuration multi-valued entry '{nameRegex}' with value regex '{valueRegex}'. Exit code '{git.ExitCode}' (level={_filterLevel})");
                 }
 
-                // TODO: don't read in all the data at once; stream it
-                string data = git.StandardOutput.ReadToEnd();
-
                 string[] entries = data.Split('\0');
                 foreach (string entry in entries)
                 {