cache.h: Introduce a generic "xdg_config_home_for(…)" function
Current implementation of `xdg_config_home(filename)` returns `$XDG_CONFIG_HOME/git/$filename`, with the `git` subdirectory inserted between the `XDG_CONFIG_HOME` environment variable and the parameter. This patch introduces a `xdg_config_home_for(subdir, filename)` function which is more generic. It only concatenates "$XDG_CONFIG_HOME", or "$HOME/.config" if the former isn’t defined, with the parameters, without adding `git` in between. `xdg_config_home(filename)` is now implemented by calling `xdg_config_home_for("git", filename)` but this new generic function can be used to compute the configuration directory of other programs. Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr> Acked-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>