meson: fix lookup of shell on MINGW64

In 4cba20f (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

meson.build

@@ -245,7 +245,7 @@ time = find_program('time', dirs: program_path, required: get_option('benchmarks
 # "/bin/sh" over a PATH-based lookup, which provides a working shell on most
 # supported systems. This path is also the default shell path used by our
 # Makefile. This lookup can be overridden via `program_path`.
-target_shell = find_program('sh', dirs: program_path + [ '/bin' ], native: false)
+target_shell = find_program('/bin/sh', 'sh', dirs: program_path, native: false)
 
 # Sanity-check that programs required for the build exist.
 foreach tool : ['cat', 'cut', 'grep', 'sort', 'tr', 'uname']