Merge log text is not correct + severity should be info and not error

Since XWIKI-9549 merge logs are now displayed when installing/upgrading an extension.

However this made 2 issues show up:

• The text displayed is not user friendly. I got this for example:

Conflict between [org.xwiki.diff.internal.ChangeDelta@8db124f0] and [org.xwiki.diff.internal.ChangeDelta@f0a39a36]$escapetool.xml($argument)]

• The log is displayed in red, which means an error for the user. But the user has merged and there's no reason to be displayed as an error. Instead it should be displayed as an information that a merge has happened.