apply --cached: fix crash in subdirectory

The static variable "prefix" was shadowed by an unused parameter
of the same name. In case of execution in a subdirectory, the
static variable was accessed, leading to a crash.
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>

builtin-apply.c

@@ -2589,7 +2589,7 @@ static int git_apply_config(const char *var, const char *value)
 }
 
 
-int cmd_apply(int argc, const char **argv, const char *prefix)
+int cmd_apply(int argc, const char **argv, const char *unused_prefix)
 {
 	int i;
 	int read_stdin = 1;