MT#7577 services: check for newly added config files

`git diff-index --name-only HEAD` was used because it was the
only reasonable way to check for modified files back in Git 1.5.
Sadly this command can't detect files that didn't exist inside
the Git repository at all yet (so if a file was generated for the
very first time the according services script wasn't executed
necessarily).

Instead nowadays we can rely on `git status --porcelain`, which
wasn't available in Git 1.5 back then yet. Even in Debian/squeeze
there's Git version 1.7.2.5-3 available, so it's save to depend
on it. Updated Git package dependency accordingly, also switch
from transitional git-core package to the actual git one.

Tested-by: Alexander Lutay <alutay@sipwise.com>