version.sh: fix bashisms

Signed-off-by: Sergei Trofimovich <slyfox@inbox.ru>

maint/version.sh

@@ -21,13 +21,11 @@ src_top_dir=
 
 VERSION_FILE=${src_top_dir}/version.h
 
-pushd ${src_top_dir} &>/dev/null
-git_head=$(git rev-parse --verify HEAD 2>/dev/null)
+git_head=$(git --git-dir "${src_top_dir}" rev-parse --verify HEAD 2>/dev/null)
 [ -z "${git_head}" ] && exit
 
-new_version="$(git describe 2>/dev/null)"
+new_version="$(git --git-dir "${src_top_dir}" describe 2>/dev/null)"
 [ -z "${new_version}" ] && exit
-popd &>/dev/null
 
 
 saved_version=