Replace shallow clone with partial clone

As recommended in https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/#user-content-quick-summary.

README.md

@@ -160,7 +160,7 @@ sudo apt install quickemu
 ## Other Linux
 
 ``` bash
-git clone --depth=1 https://github.com/wimpysworld/quickemu
+git clone --filter=blob:none https://github.com/wimpysworld/quickemu
 cd quickemu
 ```