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.
This switches fish to a "virtual" PWD, where it no longer uses getcwd to
discover its PWD but instead synthesizes it based on normalizing cd against
the $PWD variable.
Both pwd and $PWD contain the virtual path. pwd is taught about -P to
return the physical path, and -L the logical path (which is the default).
Fixes#3350
