.gitattributes & individual merge strategy for a file

Question

I have a master and a test branch of my (web) application. These projects are almost the same, except for one file which sets up the application, say "setup".

Whenever I merge one branch into the other, I would like that branch to keep its version of setup. That is, git should not attempt to merge the changes to that file.

I followed the guidance from the Pro Git book and created a .gitattributes file, with the line "setup merge=ours". However, this does not work - neither with fast forward merges not if I introduce conflicts.

(To be precise:

$: mkdir gitest
$: cd gittest
$: git init
$: echo "setup merge=ours" >> .gitattributes 
$: echo "master" >> setup
$: git add setup .gitattributes
$: git commit -a -m ...
$: git branch test
$: git checkout test
$: echo "test" >> setup
$: git commit -a -m ...
$: git checkout master
$: git merge test

Expected result: setup contains the word "master", instead git performs a ff merge and setup is "test'.)

Answer 1

I had the same error and it can be solved just defining an "ours" merge driver in .git/config:

[merge "ours"]
    name = "Keep ours merge"
    driver = true

Since true always return 0 the temporary file holding the current state will not be changed and will stay as the final version.

You can read more about merge driver in here: http://www.kernel.org/pub/software/scm/git/docs/gitattributes.html#_defining_a_custom_merge_driver

Addendum:

This works any time the driver is actually called and this seems to occur only when there are commits changing the same files (git the merge attribute). If there are changes in a single branch the driver is not going to be called.