[BUG] GIT_INDEX environment variable ignored?

Hello,

I was trying to use multiple indexes earlier, and ran into an issue which I've summarised into a test case:

$ git init $ touch file1 && git add file1 && git commit -m "file1" $ git branch release $ touch file2 && git add file2 && git commit -m "file2" $ cp .git/index .git/indexMaster $ git checkout release $ touch file3 && git add file3 && git commit -m "file3"

I then ran ls-files with the --stage option to look at what the index contains. As expected file1 and file3 are present, we're on the 'release' branch still.

$ git ls-files --stage 100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0 file1 100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0 file3

However when I run the same command again, this time using the GIT_INDEX env variable to provide the index I previously saved on master, I don't see file2 like I'd expect...

$ GIT_INDEX=.git/indexMaster git ls-files --stage 100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0 file1 100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0 file3

Is this something going wrong, or am I misunderstanding the role of the index/GIT_INDEX variable?

Replicated on 2.5.0.windows.1 and 2.6.1 (Linux).

Thanks, Ben