format-patch: replace erroneous and condition

Commit 30984ed (format-patch: support deep threading, 2009-02-19),
introduced the following lines:

	#define THREAD_SHALLOW 1

	[...]

	thread = git_config_bool(var, value) && THREAD_SHALLOW;

Since git_config_bool() returns a bool, the trailing `&& THREAD_SHALLOW`
is a no-op. Replace this errorneous and condition with a ternary
statement so that it is clear what the configured value is when a
boolean is given.

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

builtin/log.c

@@ -835,7 +835,7 @@ static int git_format_config(const char *var, const char *value, void *cb)
 			thread = THREAD_SHALLOW;
 			return 0;
 		}
-		thread = git_config_bool(var, value) && THREAD_SHALLOW;
+		thread = git_config_bool(var, value) ? THREAD_SHALLOW : THREAD_UNSET;
 		return 0;
 	}
 	if (!strcmp(var, "format.signoff")) {