Attempt to fix Windows TLS memory leak.

src/global.c

@@ -270,13 +270,21 @@ git_global_st *git__global_state(void)
 	return ptr;
 }
 
-BOOL WINAPI DllMain(HINSTANCE dll, DWORD reason, LPVOID reserved)
+void git__free_thread_global_state(void)
 {
-	if (reason == DLL_THREAD_DETACH) {
 	void *ptr = TlsGetValue(_tls_index);
+	if (!ptr)
+		return;
+
 	git__global_state_cleanup(ptr);
 	git__free(ptr);
-	}
+	TlsSetValue(_tls_index, NULL);
+}
+
+BOOL WINAPI DllMain(HINSTANCE dll, DWORD reason, LPVOID reserved)
+{
+	if (reason == DLL_THREAD_DETACH)
+		git__free_thread_global_state();
 
 	return TRUE;
 }

src/global.h

@@ -32,4 +32,6 @@ typedef void (*git_global_shutdown_fn)(void);
 
 extern void git__on_shutdown(git_global_shutdown_fn callback);
 
+extern void git__free_thread_global_state(void);
+
 #endif

src/win32/pthread.c

@@ -20,6 +20,8 @@ static DWORD WINAPI git_win32__threadproc(LPVOID lpParameter)
 
 	thread->result = thread->proc(thread->param);
 
+	git__free_thread_global_state();
+
 	return CLEAN_THREAD_EXIT;
 }