trace2: add stats for fsync operations

Add some global trace2 statistics for the number of fsyncs performed
during the lifetime of a Git process.

These stats are printed as part of trace2_cmd_exit_fl, which is
presumably where we might want to print any other cross-cutting
statistics.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -1281,6 +1281,11 @@ enum fsync_action {
  */
 int git_fsync(int fd, enum fsync_action action);
 
+/*
+ * Writes out trace statistics for fsync using the trace2 API.
+ */
+void trace_git_fsync_stats(void);
+
 /*
  * Preserves errno, prints a message, but gives no warning for ENOENT.
  * Returns 0 on success, which includes trying to unlink an object that does