I installed sublim-text using the third-party software of the software center. I did not find sublime-merge in. I know I can install it using snap, but it may be insecure.
May it be possible to integrate sublime-merge (https://www.sublimemerge.com) too to third-party softwares, please? (not a free software, I suppose licence is almost the same as sublime-text's and it is available for apt, yum). I have no idea of the work needed for this purpose. If anyone knows a better solution to deal with git repo/versions, I will be happy to have recommendations : )
Many thanks for your answers and for Solus.