ArgumentOutOfRangeException in StringBuilder.ToString

[bgrainger]

Unhandled exception: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'chunkLength')
   at System.Text.StringBuilder.ToString()
   at CloneLeeroy.Program.RunGit(String workingDirectory, String[] arguments) in /_/src/CloneLeeroy/Program.cs:line 229
   at CloneLeeroy.Program.UpdateSubmodule(String folder, String name, String branch) in /_/src/CloneLeeroy/Program.cs:line 168
   at CloneLeeroy.Program.Run(Boolean save, Boolean solutionInfoCsharp, Boolean solutionInfoHeader, String project) in /_/src/CloneLeeroy/Program.cs:line 112
   at CloneLeeroy.Program.<>c.<<Main>b__0_1>d.MoveNext() in /_/src/CloneLeeroy/Program.cs:line 53

Could presumably happen due to multithreaded access of the StringBuilder: https://stackoverflow.com/questions/38564486/system-argumentoutofrangeexception-parameter-name-chunklength-at-system-text-st

Although await process.WaitForExitAsync(); happens before the StringBuilder is accessed, it's plausible that the event handler is still running in the background.

We should solve this with a lock.