Add a workaround for using post 0.20 git versions of libgit2

Add the following to your qmake command line and things should compile
again:

qmake ... CONFIG+=libgit21-api

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add libgit2 dependency to INSTALL file (MacPorts stanza)

Signed-off-by: Giuseppe 'ferdy' Miceli <ferdy@ferdy.it>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix compile problem

QPointer was undefined (oddly I didn't run into this when test building
earlier).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>