Make git trace2 logging errors conditional on verbose mode.

Add a verbose attribute to the EventLog class, defaulting to False.
Error messages printed to sys.stderr within the EventLog.Write method
are now guarded by this verbose flag. In main.py, set EventLog.verbose
to True if the command-line --verbose option is used. This prevents
trace2 logging failures from being printed to stderr unless verbose
output is explicitly requested.

PROMPT=convert all git trace2 logging print messages to verbose only
logging

BUG: b/479811034
Change-Id: I8757ee52117d766f2f3ec47856db64cc4f51143c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/547542
Tested-by: Sam Saccone <samccone@google.com>
Reviewed-by: Julia Tuttle <juliatuttle@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

main.py

@@ -337,6 +337,9 @@ class _Repo:
             )
             return 1
 
+        cmd.CommonValidateOptions(copts, cargs)
+        git_trace2_event_log.verbose = copts.verbose
+
         if gopts.pager is not False and not isinstance(cmd, InteractiveCommand):
             config = cmd.client.globalConfig
             if gopts.pager:
@@ -359,7 +362,6 @@ class _Repo:
             Execute the subcommand.
             """
             nonlocal result
-            cmd.CommonValidateOptions(copts, cargs)
             cmd.ValidateOptions(copts, cargs)
 
             this_manifest_only = copts.this_manifest_only