Use --git-dir/--work-tree combination instead of -C

"git -C" option is only supported v1.8.5 or later. Therefore
--git-dir/--work-tree combination is more portable.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
add error check into run_browser()

Currently, vcs-ann is silently fail if w3m is not installed.
This patch adds error check.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Merge pull request #1 from kosaki/port-to-centos6

Port to centos6
(git -C is not also available on Debian squeeze.)