trace: factor out "do we want to trace" logic

As we add more tracing areas, this will avoid repeated code.

Technically, trace_printf already checks this and will avoid
printing if the trace key is not set. However, callers may
want to find out early whether or not tracing is enabled so
they can avoid doing work in the common non-trace case.

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

cache.h

@@ -1071,6 +1071,7 @@ extern void trace_vprintf(const char *key, const char *format, va_list ap);
 __attribute__((format (printf, 2, 3)))
 extern void trace_argv_printf(const char **argv, const char *format, ...);
 extern void trace_repo_setup(const char *prefix);
+extern int trace_want(const char *key);
 
 /* convert.c */
 /* returns 1 if *dst was used */

trace.c

@@ -148,10 +148,8 @@ void trace_repo_setup(const char *prefix)
 {
 	const char *git_work_tree;
 	char cwd[PATH_MAX];
-	char *trace = getenv("GIT_TRACE");
 
-	if (!trace || !strcmp(trace, "") ||
-	    !strcmp(trace, "0") || !strcasecmp(trace, "false"))
+	if (!trace_want("GIT_TRACE"))
 		return;
 
 	if (!getcwd(cwd, PATH_MAX))
@@ -168,3 +166,13 @@ void trace_repo_setup(const char *prefix)
 	trace_printf("setup: cwd: %s\n", quote_crnl(cwd));
 	trace_printf("setup: prefix: %s\n", quote_crnl(prefix));
 }
+
+int trace_want(const char *key)
+{
+	const char *trace = getenv(key);
+
+	if (!trace || !strcmp(trace, "") ||
+	    !strcmp(trace, "0") || !strcasecmp(trace, "false"))
+		return 0;
+	return 1;
+}