utils: make shell_split unicode-pure

Adjust shell_split() usage so that its inputs are always unicode
strings.  Remove the shell_usplit() function since shell_split() is now
used in its place.

Existing shell_split() callers will now do the right thing when handling
unicode.

Signed-off-by: David Aguilar <davvid@gmail.com>
grep: display "git grep" error messages

"git grep" can return non-zero exit codes.  Display its output
when this happens.

Signed-off-by: David Aguilar <davvid@gmail.com>