v.category: Address shellcheck warnings and format with shfmt

`shfmt -w -s -i 4 -ci -bn -sr .`

vector/v.category/testsuite/copy_multiple_cats.sh

@@ -9,7 +9,7 @@ set -x
 
 echo "100|100|1" | v.in.ascii output=vcategory_test_1 input=-
 fid=$(v.edit --quiet layer=1 map=vcategory_test_1 tool=select cat=1)
-v.edit map=vcategory_test_1 layer=1 tool=catadd ids=${fid} cats=2
+v.edit map=vcategory_test_1 layer=1 tool=catadd ids="${fid}" cats=2
 v.category option=transfer layer=1,2 input=vcategory_test_1 output=vcategory_test_2
 
 expected="1/2"