meson: fix lookup of shell on MINGW64

In 4cba20fbdc6 (meson: prefer shell at "/bin/sh", 2025-04-25) we have addressed an issue where the shell path embedded into Git was looked up via PATH, which easily led to unportable shell paths other than the usual "/bin/sh" location. The fix was to simply add '/bin' to the search path explicitly, which made us prefer that directory over the PATH-based lookup. This fix causes issues on MINGW64 though, which uses Windows-style paths. "/bin" is not an absolute Windows-style path, but Meson expects the directories to be absolute. This leads to the following error: meson.build:248:15: ERROR: Search directory /bin is not an absolute path. Fix this by instead searching for both '/bin/sh' and 'sh', which also causes us to prefer '/bin/sh' over a PATH-based lookup. Meson does accept that path alright on MINGW64, even though it's not an absolute Windows-style path, either. Furthermore, this continues to work alright with cross-files, as well, in case one wants to explicitly override the shell path: $ meson setup build ... Runtime executable paths perl : /nix/store/gy10hw004rl2xfbfq41vnw0yb1w8rvbl-perl-5.40.0/bin/perl python : /nix/store/sd81bvmch7njdpwx3lkjslixcbj5mivz-python3-3.13.4/bin/python3 shell : /bin/sh $ cat >cross.ini <<-EOF [binaries] sh = '/nix/store/94lg0shvsfc845zy8gnflvpqxxiyijbz-bash-interactive-5.2p37/bin/bash' EOF $ meson setup build --cross-file=cross.ini --wipe ... Runtime executable paths perl : /nix/store/gy10hw004rl2xfbfq41vnw0yb1w8rvbl-perl-5.40.0/bin/perl python : /nix/store/sd81bvmch7njdpwx3lkjslixcbj5mivz-python3-3.13.4/bin/python3 shell : /nix/store/94lg0shvsfc845zy8gnflvpqxxiyijbz-bash-interactive-5.2p37/bin/bash Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>