vim-patch.sh: Unwrap commit messages when reviewing PRs

Longer summary lines for commits may be wrapped when downloading the
commit from GitHub.  Unwrapping the lines ensures they can be compared
with the raw Vim commits.

The difference before and after this patch using:

  ./scripts/vim-patch.sh -r 8684

Before:

  ✘ Wrong commit message.
    Expected:
  vim-patch:8.0.1464: completing directory after :find does not add slash

  Problem:    Completing directory after :find does not add slash.
  Solution:   Adjust the flags for globpath(). (Genki Sky)
  vim/vim@8a37b03
    Actual:
  vim-patch:8.0.1464: completing directory after :find does not
   add slash

  Problem:    Completing directory after :find does not add slash.
  Solution:   Adjust the flags for globpath(). (Genki Sky)

After:

  ✔ Found expected commit message.

[ci skip]

1 files changed, 10 insertions, 7 deletions

diff --git a/scripts/vim-patch.sh b/scripts/vim-patch.sh
index 4912b2160b..22f946ebd9 100755
--- a/scripts/vim-patch.sh
+++ b/scripts/vim-patch.sh
@@ -449,12 +449,21 @@ review_commit() {
 
   assign_commit_details "${vim_version}"
 
+  echo
+  echo "Creating files."
+  echo "${nvim_patch}" > "${NVIM_SOURCE_DIR}/n${patch_file}"
+  msg_ok "Saved pull request diff to '${NVIM_SOURCE_DIR}/n${patch_file}'."
+  CREATED_FILES+=("${NVIM_SOURCE_DIR}/n${patch_file}")
+
+  local nvim="nvim -u NORC -n -i NONE --headless"
+  2>/dev/null $nvim --cmd 'set dir=/tmp' +'1,/^$/g/^ /-1join' +w +q "${NVIM_SOURCE_DIR}/n${patch_file}"
+
   local expected_commit_message
   expected_commit_message="$(commit_message)"
   local message_length
   message_length="$(wc -l <<< "${expected_commit_message}")"
   local commit_message
-  commit_message="$(tail -n +4 <<< "${nvim_patch}" | head -n "${message_length}")"
+  commit_message="$(tail -n +4 "${NVIM_SOURCE_DIR}/n${patch_file}" | head -n "${message_length}")"
   if [[ "${commit_message#${git_patch_prefix}}" == "${expected_commit_message}" ]]; then
     msg_ok "Found expected commit message."
   else
@@ -465,12 +474,6 @@ review_commit() {
     echo "${commit_message#${git_patch_prefix}}"
   fi
 
-  echo
-  echo "Creating files."
-  echo "${nvim_patch}" > "${NVIM_SOURCE_DIR}/n${patch_file}"
-  msg_ok "Saved pull request diff to '${NVIM_SOURCE_DIR}/n${patch_file}'."
-  CREATED_FILES+=("${NVIM_SOURCE_DIR}/n${patch_file}")
-
   get_vimpatch "${vim_version}"
   CREATED_FILES+=("${NVIM_SOURCE_DIR}/${patch_file}")