Honor default-language option

[dandavison]

Fixes https://github.com/dandavison/delta/discussions/858

The default-language option had been introduced initially in order to determine language for git blame (where there are no file names). However, subsequently git blame came to determine the language by inspecting the command line of the calling process (e.g. if delta sees the calling process is git blame src/file.ts then the language is typescript). Meanwhile default-language acquired its obvious function of providing a default language for diffs; this PR fixes that.

Note that the best place to set default-language is probably in your per-repository git settings. I.e. $my_repo/.git/config.