### Please describe the problem.

After unlocking a large file and editing it, the attempt to commit the change fails like so:

git-annex: fd:16: hGetBuf: invalid argument (Invalid argument)

### What steps will reproduce the problem?

(it needs to be over 2G in size to reproduce the problem)

### What version of git-annex are you using? On what operating system?

git-annex version: 6.20160923 on macOS 10.12

### Please provide any additional information below.

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log

$ dd if=/dev/zero of=largefile.txt bs=1048576 count=2048
$ git annex add largefile.txt
$ git commit -m 'added large file'
$ git annex unlock largefile.txt
$ echo foobar >> largefile.txt
$ git commit -a -m 'edited large file'
git-annex: fd:16: hGetBuf: invalid argument (Invalid argument)

# End of transcript or log.
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

> reproduced; and confirmed that the fix below really did fix it on OSX.
> [[done]] --[[Joey]]
