This is a repo that demonstrates a problem with git 2.7.2, 2.8.x and 2.9.x merging. It seems to have been fixed in 2.10.x. You can see the error by running:

$ git merge merge_me

    error: addinfo_cache failed for path 'README.txt'
    Auto-merging README.txt
    README.txt: unmerged (f00c965d8307308469e537302baa73048488f162)
    README.txt: unmerged (133197178b0ef0b23c59977de823fd4ffc21025d)
    README.txt: unmerged (3207e44cfb484ba4214df3ee949b5570048cf436)
    fatal: git write-tree failed to write a tree

You need to make sure that you have core.autocrlf = false

The master branch has a file called README.txt. This file contains lines that end with \n (Unix) and some that end in \r\n (Windows). The merge_me branch contains a version of README.txt that converted all the EOL to \n. The branch also has a .gitattributes file that makes git do EOL conversion for all text files:

# Auto detect text files and perform LF normalization
* text=auto