format-patch: replace erroneous and condition

format-patch: replace erroneous and condition

Commit 30984ed2e9 (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 <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>