chore(vim-patch.sh): use piping instead of here string for while read

Using a here string can cause an error if there are no missing patches:
`./scripts/vim-patch.sh: line 580: runtime_commits: bad array subscript`

Using piping doesn't cause the error.

scripts/vim-patch.sh

@@ -576,13 +576,13 @@ show_vimpatches() {
 # Prints a human-formatted list of Vim commits, with instructional messages.
 # Passes "$@" onto list_missing_vimpatches (args for git-log).
 show_vimpatches() {
-  get_vim_sources update
+  get_vim_sources update
   printf "Vim patches missing from Neovim:\n"
 
   local -A runtime_commits
   for commit in $(git -C "${VIM_SOURCE_DIR}" log --format="%H %D" -- runtime | sed 's/,\? tag: / /g'); do
     runtime_commits[$commit]=1
-  done
+  done
 
   list_missing_vimpatches 1 "$@" | while read -r vim_commit; do
     if [[ "${runtime_commits[$vim_commit]-}" ]]; then