Inconsistent path separators when trying to list paths using `doc --help --verbose` on mingw

[Teapot4195]

When using ./x.py doc --help --verbose (this probably happens for everything else that might list paths on mingw). I expected it to output a list of paths using just / or just \ as separator, instead I get

    ./x.py doc src/doc
    ./x.py doc src/doc/book
    ./x.py doc src/doc/edition-guide
    ./x.py doc src/doc/embedded-book
    ./x.py doc src/doc/nomicon
    ./x.py doc src/doc/reference
    ./x.py doc src/doc/rust-by-example
    ./x.py doc src/doc/rustc
    ./x.py doc src/doc/rustdoc
    ./x.py doc src/doc/style-guide
    ./x.py doc src/doc/unstable-book
    ./x.py doc src\librustdoc
    ./x.py doc src\rustdoc-json-types
    ./x.py doc src\tools\cargo
    ./x.py doc src\tools\cargo\crates\cargo-platform
    ./x.py doc src\tools\cargo\crates\cargo-test-macro
    ./x.py doc src\tools\cargo\crates\cargo-test-support
    ./x.py doc src\tools\cargo\crates\cargo-util
    ./x.py doc src\tools\cargo\crates\crates-io
    ./x.py doc src/tools/cargo/src/doc
    ./x.py doc src\tools\clippy
    ./x.py doc src/tools/clippy/book
    ./x.py doc src\tools\clippy\clippy_lints
    ./x.py doc src\tools\clippy\clippy_utils
    ./x.py doc src\tools\clippy\declare_clippy_lint
    ./x.py doc src/tools/error_index_generator
    ./x.py doc src\tools\miri
    ./x.py doc src\tools\miropt-test-tools
    ./x.py doc src\tools\rustdoc
    ./x.py doc src\tools\rustfmt
    ./x.py doc src\tools\rustfmt\config_proc_macro
    ./x.py doc src\tools\tidy
    ./x.py doc src/tools/unstable-book-gen
    ./x.py doc standalone

in addition, just copypasting those that use backslashes don't work out of the box as they are treated as escapes by bash. Ideally, we would use all slashes or all backslashes and double the backslashes so that the line can be directly copypasted into the terminal

@rustbot label +T-bootstrap

[fmease]

@rustbot label C-enhancement A-contributor-roadblock

[ChrisDenton]

On native shells \ should be used consistently and not escaped.

Support for non-native shells like msys2 would need some way to know to prefer cygwin-style paths. Maybe the preference could be implemented via a setting or an environment variable. In any case, cygpath could perhaps be used for this and other related msys2 path issues.

[jyn514]

@rustbot label O-windows