git merge: Removing files I want to keep!

This is an interesting issue. Because you deleted the file after BranchA was created, and then are merging master into BranchA, I'm not sure how Git would be able to realize there is a conflict.

After the bad merge you can undo, and then re-merge, but add back the file:

git checkout HEAD@{1} .
git merge --no-commit master
git checkout master test.txt
git add test.txt
git commit

Casey's example didn't work for my case - I couldn't checkout test.txt from master, because it was no longer in that branch:

$ git checkout master test.txt
error: pathspec 'test.txt' did not match any file(s) known to git.

Happily I could pull the file out of branchA's own HEAD:

$ git checkout branchA
$ git merge --no-commit master
$ git checkout HEAD test.txt
$ git add test.txt
$ git commit