'status' in fix mode does not successfully fix

Not sure if I'm doing something wrong, or it's a bug:

```
$ git-crypt --version
git-crypt 0.5.0

$ git --version
git version 2.6.1

$ cd /tmp && mkdir test && cd test && git init
Initialized empty Git repository in /tmp/test/.git/

$ git-crypt init
Generating key...

$ git-crypt add-gpg-user scott@smemsh.net
[master (root-commit) fe5dae3] Add 1 git-crypt collaborator
2 files changed, 3 insertions(+)
create mode 100644 .git-crypt/.gitattributes
create mode 100644 .git-crypt/keys/default/0/B2CE7E00F1C9C1AB19D835E5F95422EF3AED5886.gpg

$ echo '.gitattributes !filter !diff' > .gitattributes &&
echo 'secret1 filter=git-crypt diff=git-crypt' >> .gitattributes &&
touch secret1 && git add secret1 && git ci -m test1
[master 3e3182c] test1
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 secret1

$ git-crypt lock

$ echo 'secret2 filter=git-crypt diff=git-crypt' >> .gitattributes &&
touch secret2 &&
git add secret2 &&
git-crypt status
not encrypted: .gitattributes
not encrypted: .git-crypt/.gitattributes
not encrypted: .git-crypt/keys/default/0/B2CE7E00F1C9C1AB19D835E5F95422EF3AED5886.gpg
encrypted: secret1
encrypted: secret2 *** WARNING: staged/committed version is NOT ENCRYPTED! ***

Warning: one or more files is marked for encryption via .gitattributes but
was staged and/or committed before the .gitattributes file was in effect.
Run 'git-crypt status' with the '-f' option to stage an encrypted version.
```

ok so at this point is when I said "oh no" and went to fix it:

```
$ git-crypt status -f
Error: secret2: still unencrypted even after staging
Unable to stage 1 file.
```

I saw bug #62 and #63; this appears to be similar, but I made
a new bug since I include all reproduction steps, and that
one looked like there was some possibly unknown factors
leading up to it.

If this happens because it's locked (as I suspect), then the error
message should say so, since _git-crypt_ has this information.