Git revision and branch computed only once

Previously, the CMake script 'getgit.cmake' was invoked multiple times
to generate header files containing preprocessor definitions containg
information about the Git revision in case KBibTeX's sources came
straight from a Git checkout.
This script is run only once now by introducing the indirection of a
custom target that depends on the header files as generated by the
script; this custom target in its turn is a dependency for several of
KBibTeX's libraries and programs.

Version information printed on console via debug output is now taken
from a KAboutData instance, i.e. the same as shown in the About dialog.

Also: revamping, modernizing, reformatting, and refactoring CMake
script 'getgit.cmake'.