Add variables diffext and diffexe?

[muzimuzhi]

Currently the comparison method can only be configured through environment variables diffext and diffexe. Considering their short names (compared to e.g., L3BUILD_CONFIG_DIFFEXT and L3BUILD_CONFIG_DIFFEXE), I hesitate to put them in the shell profile(s).

Maybe they can also be provided as same-named l3build variables?

BTW what I want is to use git diff as the diff program, as I'm more used to its format:

diffext = ".diff"
diffexe = "git diff --no-index --text --ignore-cr-at-eol -- "