[PATCH] Unset variables messing with Git use

[martin f. krafft]

This patch makes sure that variables from the environment cannot
override e.g. the Git directory to operate on. Without it, in a setting
where GIT_DIR is set, pass would otherwise commit generated password
files to the wrong repository.

I caught this, because fortunately the other repository had a .gitignore
that would have ignored the file:

```
fishbowl~% echo $GIT_DIR
/home/madduck/.config/vcsh/repo.d/zsh.git

fishbowl~% pass generate test
The following paths are ignored by one of your .gitignore files:
.password-store/test.gpg
Use -f if you really want to add them.
The generated password for test is:
…
```

The result was an orphan file `test.gpg` in the password-store root.

Signed-off-by: martin f. krafft <madduck at madduck.net>
---
 src/password-store.sh | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/password-store.sh b/src/password-store.sh
index 1d119f2..d3b0432 100755
--- a/src/password-store.sh
+++ b/src/password-store.sh
@@ -20,6 +20,8 @@ GENERATED_LENGTH="${PASSWORD_STORE_GENERATED_LENGTH:-25}"
 CHARACTER_SET="${PASSWORD_STORE_CHARACTER_SET:-[:punct:][:alnum:]}"
 CHARACTER_SET_NO_SYMBOLS="${PASSWORD_STORE_CHARACTER_SET_NO_SYMBOLS:-[:alnum:]}"
 
+unset GIT_DIR GIT_WORK_TREE GIT_NAMESPACE GIT_INDEX_FILE GIT_OBJECT_DIRECTORY
+
 export GIT_CEILING_DIRECTORIES="$PREFIX/.."
 
 #
-- 
2.23.0.rc1