[commit] Change merge commit message to be valid Markdown

[aik099]

The Phabricator tool, that shows commits is parsing commit messages using it's own Markdown-alike parser. The currently generated merge commit message looks very bad, when rendered:

The following must be attempted (only for commit with 2+ lines):

• if 2nd line of message present and doesn't look like list (1. or - or *), then wrap 2nd+ till commit message end line using Fenced Code Block
• otherwise ident using each line after 1st one using 1 space