"add -p" + filenames with UTF-8 multibyte characters = "No changes"

Hello all,

I seem to have run into a bug in "add -p" and "add -i" when trying to
stage diff hunks in tracked files with UTF-8 multibyte characters
(such as "á"). If I add "á", commit, then modify it and try to run
"add -p" on it, Git reports "No changes". "add -i" doesn't do
anything, either.

I've switched to 1.6.2.rc0.90.g0753 and the problem persists. If it
helps, I've attached a small shell script with a minimal recipe for
triggering the bug.

This should incorrectly report "No changes":
./test-accents.sh

And this should work fine:
FILE=a ./test-accents.sh

-Antonio