chore(merge-script): add a way to add optional merge message

This allows to close related to PR issues when PR submitter forgets to
include that in commit message, or add some other additional info.

E.g. `./merge-pr.sh 3033 'Fu fu fu. :3'` would result in:
```
    Merge pull request #3033

    Fu fu fu. :3

    sudden6 (1):
          update friend activity on groupchat events     fixes #2848
```

merge-pr.sh

@@ -21,34 +21,57 @@
 # Requires SSH key that github accepts and GPG set to sign merge commits.
 
 # usage:
-#   ./$script $pr_number
+#   ./$script $pr_number $optional_message
+#
+#
+# $pr_number – number of the PR as shown on GH
+# $optional_message – message that is going to be put in merge commit,
+#       before the appended shortlog.
+#
 
 PR=$1
 
+# make sure to add newlines to the message, otherwise merge message
+# will not look well
+if [[ ! -z $2 ]]
+then
+    OPT_MSG="
+$2
+"
+fi
+
+
 # FIXME: ↓
 #[[ ${PR} =~ "^[:digit:]+$" ]] || echo "Not a PR number!" && exit 1
 
 # list remotes, and if there's no tux3 one, add it
-if  ! git remote | grep original
+if  ! git remote | grep original > /dev/null
 then
     git remote add original git@github.com:tux3/qTox.git
 fi
 
+# print the message only if the merge was successful
+after_merge_msg() {
+    echo ""
+    echo "PR #$PR was merged into «merge$PR» branch."
+    echo "To compare with master:"
+    echo "git diff master..merge$PR"
+    echo ""
+    echo "To push that to master on github:"
+    echo "git checkout master && git merge --ff merge$PR && git push original master"
+    echo ""
+    echo "After pushing to master, delete branches:"
+    echo ""
+    echo "git branch -d {merge,}$PR"
+}
+
+
 git fetch original && \
 git checkout master && \
 git rebase original/master master && \
 git fetch original pull/$PR/head:$PR && \
 git checkout master -b merge$PR && \
 git merge --no-ff -S $PR -m "Merge pull request #$PR
-
+$OPT_MSG
 $(git shortlog master..$PR)" && \
-echo ""
-echo "PR #$PR was merged into «merge$PR» branch."
-echo "To compare with master:"
-echo "git diff master..merge$PR"
-echo ""
-echo "To push that to master on github:"
-echo "git checkout master && git merge --ff merge$PR && git push original master"
-echo ""
-echo "After pushing to master, delete branches:"
-echo "git branch -d {merge,}$PR"
+after_merge_msg