CHANGELOG.md: update to new format

Update commit message entries to new format, by making commit sha
monospace and bold so that they take the same width in the list.

Also, make all commit messages be surrounded by ` to show them in
monospace as well; they might not always include English words but
identifiers and abbreviations instead.

Vim substitutions used:

- %s/- [[]\(........\)[]]/- \[**`\1`**\]/

Signed-off-by: Manos Pitsidianakis <manos@pitsidianak.is>