trace2: write normal format events in snake case

A previous patch [1] added the ability to write strings in snake case.
However, this was not applied to writing events in the normal format -
events that should be written as child_start or child_exit are currently
written as childstart and childexit. This patch corrects this issue.

[1]: 7d2a8cc

Author: ldennington

src/shared/Core/Trace2.cs

@@ -467,7 +467,7 @@ protected string BuildNormalString(string message)
 
         // Git's TRACE2 normal format is:
         // [<time> SP <filename>:<line> SP+] <event-name> [[SP] <event-message>] LF
-        return $"{time} {source,-33} {Event.ToString().ToLower()} {message}";
+        return $"{time} {source,-33} {Event.ToString().ToSnakeCase()} {message}";
     }
 }