fix on OSX

set -e causes the xecho to not run if the xgrep fails. At least with OSX's
/bin/sh, which is:

GNU bash, version 3.2.51(1)-release (x86_64-apple-darwin13)

This didn't happen on Linux with:

GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu)

Possibly a bug in bash, or an OSX-specific bug. However, disabling set -e
in the subshell seems a good idea anyway.

fixes https://github.com/blake2-ppc/git-remote-gcrypt/issues/15