Added a script to bump PORTREVISION if a library has changed a version like libgit2.

Just execute it in the root of you ports with e.g.
Tools/scripts/search_lib_depends_and_bump.sh devel/libgit2

This will search over all existing ports, check if they need to be bumped.
After all ports are bumped it triggers a portlint for each modified port.
The portlint loop continues till all errors and warnings are fixed.
Feel free to break this loop using CTRL+C.