ThreadSanitizer: add suppressions

Add a file .tsan-suppressions and list two functions in it: want_color()
and transfer_debug(). Both of these use the pattern

	static int foo = -1;
	if (foo < 0)
		foo = bar();

where bar always returns the same non-negative value. This can cause
ThreadSanitizer to diagnose a race when foo is written from two threads.
That is indeed a race, although it arguably doesn't matter in practice
since it's always the same value that is written.

Add NEEDSWORK-comments to the functions so that this problem is not
forever swept way under the carpet.

The suppressions-file is used by setting the environment variable
TSAN_OPTIONS to, e.g., "suppressions=$(pwd)/.tsan-suppressions". Observe
that relative paths such as ".tsan-suppressions" might not work.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

color.c

@@ -338,6 +338,13 @@ static int check_auto_color(void)
 
 int want_color(int var)
 {
+	/*
+	 * NEEDSWORK: This function is sometimes used from multiple threads, and
+	 * we end up using want_auto racily. That "should not matter" since
+	 * we always write the same value, but it's still wrong. This function
+	 * is listed in .tsan-suppressions for the time being.
+	 */
+
 	static int want_auto = -1;
 
 	if (var < 0)