trace: give repo_setup trace its own key

You no longer get this output with GIT_TRACE=1; instead, you
can do GIT_TRACE_SETUP=1.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1510-repo-setup.sh

@@ -57,7 +57,7 @@ test_repo () {
 			export GIT_WORK_TREE
 		fi &&
 		rm -f trace &&
-		GIT_TRACE="$(pwd)/trace" git symbolic-ref HEAD >/dev/null &&
+		GIT_TRACE_SETUP="$(pwd)/trace" git symbolic-ref HEAD >/dev/null &&
 		grep '^setup: ' trace >result &&
 		test_cmp expected result
 	)

trace.c

@@ -155,10 +155,11 @@ static const char *quote_crnl(const char *path)
 /* FIXME: move prefix to startup_info struct and get rid of this arg */
 void trace_repo_setup(const char *prefix)
 {
+	static const char *key = "GIT_TRACE_SETUP";
 	const char *git_work_tree;
 	char cwd[PATH_MAX];
 
-	if (!trace_want("GIT_TRACE"))
+	if (!trace_want(key))
 		return;
 
 	if (!getcwd(cwd, PATH_MAX))
@@ -170,10 +171,10 @@ void trace_repo_setup(const char *prefix)
 	if (!prefix)
 		prefix = "(null)";
 
-	trace_printf("setup: git_dir: %s\n", quote_crnl(get_git_dir()));
-	trace_printf("setup: worktree: %s\n", quote_crnl(git_work_tree));
-	trace_printf("setup: cwd: %s\n", quote_crnl(cwd));
-	trace_printf("setup: prefix: %s\n", quote_crnl(prefix));
+	trace_printf_key(key, "setup: git_dir: %s\n", quote_crnl(get_git_dir()));
+	trace_printf_key(key, "setup: worktree: %s\n", quote_crnl(git_work_tree));
+	trace_printf_key(key, "setup: cwd: %s\n", quote_crnl(cwd));
+	trace_printf_key(key, "setup: prefix: %s\n", quote_crnl(prefix));
 }
 
 int trace_want(const char *key)