KDEGitCommitHooks: Allow passing in of custom scripts

In case of kphotoalbum, the maintainer wants the clang-format hook in
addition to a set of custom hooks. While we currently append to the file
in case it exists, it would still mean that custom hooks have to be
installed/configured separately.

Because developers can not specify custom hooks, we can just override
the file which will make the entire generation process easier.
Since this is a small behavior change, it is put in a version guard.

BUG: 472361
FIXED-IN: 5.109