Visual Studio 2012 doesn't have allow the 'compare' command when merging changes from one branch to another.

Here is the uservoice request to add the feature to VS 2012.  If you know me please vote for it.  :)
http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/2947191-always-allow-compare-in-pending-changes-for-br


It does allow it when VS is configured to use an external compare tool.  My favorite is SourceGear DiffMerge (free).

Merge
/m /r=%4 /t1=%7 /t2=%8 /t3=%6 /c=%9 %2 %3 %1

Compare
/t1=%6 /t2=%7 %1 %2

Best feature: this tool also makes is brain-dead simple to move chunks of code (if you change your mind about checking in that web.config change at the last minute).  I'm really surprised VS didn't copy this feature from all the OSS merge tools.  They did all the work to have VS keyboard shortcuts and syntax highlighting, but didn't add 'move this block from 'Server' version to 'Local'.  This is now (finally) possible, but involves copy/paste and disrupts flow when reviewing large merges and large check-ins.