chore: actually restrict length the first line of `Merge*` commits

Regexp didn't actually restrict it, found by @iphydf.

Verification now treats commit & merge messages differently, since as
pointed out by @initramfs treating them the same could result in marking
as valid a non-merge commit that starts with `Merge`.

`s/Pls/Please/` as requested by @initramfs.

Also require a space between `Merge` word and rest of the commit
message.

verify-commit-messages.sh

@@ -15,8 +15,8 @@
 #    You should have received a copy of the GNU General Public License
 #    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-# Script for verifying conformance to commit message format of commits in commit
-# range supplied.
+# Script for verifying conformance to commit message format of commits in
+# commit range supplied.
 #
 # Script fails (non-zero exit status) if commit messages don't conform.
 
@@ -25,19 +25,30 @@
 #
 # $commit_range – in format `abdce..12345`
 
+
+# Fail as soon as an error appears
+set -eu -o pipefail
+
 ARG="$1"
 
 echo "" # ← formatting
 
+grep_for_invalid() {
+    # differentiate what is allowed for commit messages and merge messages
+    git log --no-merges --format=format:'%s' "$ARG" \
+        | grep -v -E '^(feat|fix|docs|style|refactor|perf|revert|test|chore)(\(.{,12}\))?:.{1,68}$' \
+        || git log --merges --format=format:'%s' "$ARG" \
+            | grep -v -E '^Merge .{1,70}$'
+}
+
 # Conform, /OR ELSE/.
-if git log --format=format:'%s' "$ARG" | \
-    grep -v -E '^((feat|fix|docs|style|refactor|perf|revert|test|chore)(\(.{,12}\))?:.{1,68})|(Merge(.+){,70})$'
+if grep_for_invalid
 then
     echo ""
     echo "Above ↑ commits don't conform to commit message format:"
     echo "https://github.com/qTox/qTox/blob/master/CONTRIBUTING.md#commit-message-format"
     echo ""
-    echo "Pls fix."
+    echo "Please fix."
     echo ""
     echo "If you're not sure how to rewrite history, here's a helpful tutorial:"
     echo "https://www.atlassian.com/git/tutorials/rewriting-history/git-commit--amend/"