BUG: git prompt breaks with zsh < 5.0.6

["Shupak, Vitaly" <Vitaly.Shupak@xxxxxxxxxx>]

A change to git-prompt.sh in v2.38 to show presence of unresolved conflicts contains syntax which is not valid under zsh < 5.0.6. This version of zsh is old, but is still the default on RedHat 7 machines, for example.

The offending block is:

       local conflict="" # state indicator for unresolved conflicts
       if [[ "${GIT_PS1_SHOWCONFLICTSTATE}" == "yes" ]] &&
          [[ $(git ls-files --unmerged 2>/dev/null) ]]; then
               conflict="|CONFLICT"
       fi

[[ $(command) ]] syntax doesn't work in older version of zsh and should use [[ -n "$(command)" ]] instead.

Thanks,
Vitaly