Made sed expression GNU and BSD compatible.

A recent change in pass.zsh-completion broke autocompletion when using
BSD sed. I’ve made the relevant sed expression compatible with GNU and
BSD sed.

Previous change with regression:
http://lists.zx2c4.com/pipermail/password-store/2014-April/000773.html

http://git.zx2c4.com/password-store/commit/src/completion/pass.zsh-completion?id=f82e9d6cf3bc3a12bdfce89bf319d76f79e66efc

From Kevin:

I found a small bug in the zsh completions.  Basically when the
PASSWORD_STORE_DIR ends in a slash the first character of the
result is eaten, making completion essentially useless. (It does
this before determining matches).

This can be fixed by changing what is line 106 in my version from:

_values -C 'passwords' $(find -L "$prefix" \( -name .git -o -name
.gpg-id \) -prune -o $@ -print | sed -e "s#${prefix}.##" -e 's#\.gpg##'
| sort)

to

_values -C 'passwords' $(find -L "$prefix" \( -name .git -o -name
.gpg-id \) -prune -o $@ -print | sed -e "s#${prefix}/\\?##" -e
's#\.gpg##' | sort)

The difference is the first sed regex expression.  The original
version assumed that the next character was a slash and removed
it while the new version only removes it if it is a slash.

"s#${prefix}.##" -> "s#${prefix}/\\?##"

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reported-by: Kevin Cox <kevincox@kevincox.ca>

Based-on-work-by: Matthieu Weber <mweber@free.fr>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

This relies on a patched version of tree to work, unfortunately.
Hopefully upstream will accept our patch.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>