This program will use the 3 provided versions of the Debian changelog to generate a merged changelog file. The resulting changelog is stored in the file out or output to the standard output if that parameter is not given.
Each entry is identified by its version number and they are assumed to be not conflicting, they are simply merged in the right order (by decreasing version number). When --merge-prereleases is used, the part of the version number after the last tilde is dropped so that 1.0-1~exp1 and 1.0-1~exp5 are considered to be the same entry. When the same version is available in both new-a and new-b, a standard line-based 3-way merge is attempted (provided that the module Algorithm::Merge is available --- it's part of the package libalgorithm-merge-perl --- otherwise you get a global conflict on the content of the entry).
This is useful when you keep using the same changelog entry but you increase its version number regularly. For instance, you might have 2.3-1~exp1, 2.3-1~exp2, ... until the official release 2.3-1 and they are all the same changelog entry that has evolved over time.
If you want to use this program to merge Debian changelog files in a git repository, you have first to register a new merge driver in .git/config or ~/.gitconfig:
name = debian/changelog merge driver
driver = dpkg-mergechangelogs -m %O %A %B %A
Then you have to setup the merge attribute for the debian/changelog file either in .gitattributes in the repository itself, or in .git/info/attributes: