Every builtin or function shipped with fish supports flag -h or --help to
print a slightly condensed version of its manpage.
Some of those help messages are longer than a typical screen;
this commit pipes the help to a pager to make it easier to read.
As in other places in fish we assume that either $PAGER or "less" is a
valid pager and use that.
In three places (error messages for bg, break and continue) the help is
printed to stderr instead of stdout. To make sure the error message is
visible in the pager, we pass it to builtin_print_help, every call of which
needs to be updated.
Fixes#6227
This runs build_tools/style.fish, which runs clang-format on C++, fish_indent on fish and (new) black on python.
If anything is wrong with the formatting, we should fix the tools, but automated formatting is worth it.
realpath() will return NULL and sets errno if it fails.
We asserted that realpath(".") does not fail. We also didn't really
check that it was successful. Made sure we'll get a perror telling
us about what went wrong if something like this happens again.
Updated tests and added test case
Fixes#5351
