reveal command is failed in the sub directory

[ochanism]

#751
This PR produced this bug.

• How to reproduce

$ git init
Initialized empty Git repository in /home/jayden/Workspace/opensource/test/.git/

$ git-secret init
git-secret: init created: '/home/jayden/Workspace/opensource/test/.gitsecret/'

$ git-secret tell test@test.com
gpg: keybox '/home/jayden/Workspace/opensource/test/.gitsecret/keys/pubring.kbx' created
gpg: /home/jayden/Workspace/opensource/test/.gitsecret/keys/trustdb.gpg: trustdb created
git-secret: done. test@test.com added as user(s) who know the secret.

$ touch aaa  

$ git-secret add aaa
git-secret: file not in .gitignore, adding: aaa
git-secret: 1 item(s) added.

$ git-secret hide
git-secret: done. 1 of 1 files are hidden.

# `reveal` worked well in the git repo root directory.
$ git-secret reveal
File '/home/jayden/Workspace/opensource/test/aaa' exists. Overwrite? (y/N) y
git-secret: done. 1 of 1 files are revealed.

$ mkdir subdir

$ cd subdir 

# `reveal` failed in the subdir.
$ git-secret reveal
git-secret: abort: cannot find file to decrypt: /home/jayden/Workspace/opensource/test/subdir/aaa.secret

The file path should be /home/jayden/Workspace/opensource/test/aaa.secret instead of /home/jayden/Workspace/opensource/test/subdir/aaa.secret.

[ochanism]

@sobolevn
I've made a patch for this bug.
ochanism@306fa6f

If this is acceptable, I'll make a PR.

[sobolevn]

Please, do :)

[ochanism]

Please review this PR: #905
I hope it would be released asap because I've been suffering from this bug. 😄