Include Merge description message in merge by default. User can disable description in merge message by clicking on `Don't include description in commit message`.

Drop the merge title which is "Merge branch SOURCE into TARGET" and only keep the MR title as commit message.

mimemagic was yanked from rubygems, so this cause gitlab to fail building. To
make it possible to keep installing this old version of gitlab we get mimemagic
from git.

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