XWIKI-9549: Merge log does not end up in the XAR extension install log
* Iterate over the list of message elements rather than the list of arguments because the arguments cannot always be trusted.
* Improve a bit the display of diff Delta in merge log. E.g.: Conflict between [@@ -1,1 +1,1 @@] and [@@ -1,1 +1,1 @@]