docs: fix filter-branch subdir example for exotic repo names

The GIT_INDEX_FILE variable we get from git has the full
path to the repo, which may contain spaces. When we use it
in our shell snippet, it needs to be quoted.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -361,7 +361,7 @@ git filter-branch --index-filter \
 	'git ls-files -s | sed "s-\t\"*-&newsubdir/-" |
 		GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
 			git update-index --index-info &&
-	 mv $GIT_INDEX_FILE.new $GIT_INDEX_FILE' HEAD
+	 mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"' HEAD
 ---------------------------------------------------------------