chore: fix a compilation bug on MacOS and update .gitignore

.gitignore

@@ -17,3 +17,7 @@ kernelmemfs
 mkfs
 .gdbinit
 .gdb_history
+.depend
+.history/
+.vscode/
+cscope.out

sh.c

@@ -53,6 +53,8 @@ int fork1(void);  // Fork but panics on failure.
 void panic(char*);
 struct cmd *parsecmd(char*);
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Winfinite-recursion"
 // Execute cmd.  Never returns.
 void
 runcmd(struct cmd *cmd)
@@ -129,6 +131,7 @@ runcmd(struct cmd *cmd)
   }
   exit();
 }
+#pragma GCC diagnostic pop
 
 int
 getcmd(char *buf, int nbuf)

usertests.c

@@ -282,7 +282,7 @@ exectest(void)
 }
 
 void
-nullptr(void)
+nullptrtest(void)
 {
   printf(1, "null pointer test\n");
   printf(1, "expect one killed process\n");
@@ -1648,7 +1648,7 @@ main(int argc, char *argv[])
   pipe1();
   preempt();
   exitwait();
-  nullptr();
+  nullptrtest();
 
   rmdot();
   fourteen();