Update documentation for git diff

Author: HE7086

README.md

@@ -181,7 +181,7 @@ You can combine `bat` with `git diff` to view lines around code changes with pro
 highlighting:
 ```bash
 batdiff() {
-    git diff --name-only --relative --diff-filter=d | xargs bat --diff
+    git diff --name-only --relative --diff-filter=d -z | xargs --null bat --diff
 }
 ```
 If you prefer to use this as a separate tool, check out `batdiff` in [`bat-extras`](https://github.com/eth-p/bat-extras).

doc/README-ko.md

@@ -163,7 +163,7 @@ git show v0.6.0:src/main.rs | bat -l rs
 볼 수 있습니다:
 ```bash
 batdiff() {
-    git diff --name-only --diff-filter=d | xargs bat --diff
+    git diff --name-only --diff-filter=d -z | xargs --null bat --diff
 }
 ```
 이것을 별도의 도구로 쓰고 싶다면

doc/README-zh.md

@@ -150,7 +150,7 @@ git show v0.6.0:src/main.rs | bat -l rs
 
 ```bash
 batdiff() {
-    git diff --name-only --diff-filter=d | xargs bat --diff
+    git diff --name-only --diff-filter=d -z | xargs --null bat --diff
 }
 ```