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