var: do not print usage() with a correct invocation

Before, git-var could print usage() even if the command was invoked
correctly with a variable defined in git_vars -- provided that its
read() function returned NULL.

Now, we only print usage() only if it was called with a logical
variable that wasn't defined -- regardless of read().

Since we now know the variable is valid when we call read_var(), we
can avoid printing usage() here (and exiting with code 129) and
instead exit quietly with code 1. While exiting with a different code
can be a breaking change, it's far better than changing the exit
status more generally from 'failure' to 'success'.

Signed-off-by: Sean Allred <allred.sean@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-var.txt

@@ -13,7 +13,8 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Prints a Git logical variable.
+Prints a Git logical variable. Exits with code 1 if the variable has
+no value.
 
 OPTIONS
 -------

builtin/var.c

@@ -56,18 +56,15 @@ static void list_vars(void)
 			printf("%s=%s\n", ptr->name, val);
 }
 
-static const char *read_var(const char *var)
+static const struct git_var *get_git_var(const char *var)
 {
 	struct git_var *ptr;
-	const char *val;
-	val = NULL;
 	for (ptr = git_vars; ptr->read; ptr++) {
 		if (strcmp(var, ptr->name) == 0) {
-			val = ptr->read(IDENT_STRICT);
-			break;
+			return ptr;
 		}
 	}
-	return val;
+	return NULL;
 }
 
 static int show_config(const char *var, const char *value, void *cb)
@@ -81,7 +78,9 @@ static int show_config(const char *var, const char *value, void *cb)
 
 int cmd_var(int argc, const char **argv, const char *prefix)
 {
-	const char *val = NULL;
+	const struct git_var *git_var;
+	const char *val;
+
 	if (argc != 2)
 		usage(var_usage);
 
@@ -91,10 +90,15 @@ int cmd_var(int argc, const char **argv, const char *prefix)
 		return 0;
 	}
 	git_config(git_default_config, NULL);
-	val = read_var(argv[1]);
-	if (!val)
+
+	git_var = get_git_var(argv[1]);
+	if (!git_var)
 		usage(var_usage);
 
+	val = git_var->read(IDENT_STRICT);
+	if (!val)
+		return 1;
+
 	printf("%s\n", val);
 
 	return 0;