Upgrade libgit2

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Support diff.context config key

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Merge branch 'diff.context'
Do not show view range log menu items if one of the selected revisions is working dir changes

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>