Merge without marking conflicts in working tree

[Aaron Schrab <aaron@schrab.com>]

Is there a way to do a merge but only record conflicts in the index, not 
update the working versions of files with conflict markers?

Like many people, I use git to manage configuration files for my shell, 
editor, git itself, and a number of other things.  The vast majority of 
times that I update things no conflicts are occur and everything just 
works, so I'd like to avoid extra work in this case.  But occasionally a 
conflict will occur, and if it's in a file that will be read while 
trying to resolve the conflict this can make things more difficult.

I'd like to find a way to have the conflict recorded in just the index 
without touching the working tree.  I could then use my usual tools to 
resolve the conflict without the errors caused by the conflict markers.  
I generally use vim+fugitive to resolve conflicts anyway, and typically 
the first step I take is to replace the working-tree version with the 
merge-base version, completely ignoring any conflict markers.

If there isn't currently a way to do this, I was thinking of 
implementing something like an "ours" value for merge.conflictstyle 
configuration.