fix(option): set 'shell…' options based on detected shell #40031

Problem:
* 'shellcmdflag' states that its default value is set according to the
  value of 'shell', but this behavior is not yet implemented on Windows.
  The same applies to 'shellpipe', 'shellredir', and 'shellxquote'.
* On Windows, Git is often installed in paths containing spaces, and we
  still do not correctly resolve the sh executable name as described in
  'shell'.
* On Windows, the default value of 'shellslash' is always `false`,
  which causes Unix-like shells to interpret `\` in paths returned by
  some functions as escape charaters.

Solution:
Use a simple rule table to detect common shells (e.g. `cmd`,
`powershell`, shells whose names contain `csh` or `sh`) and apply
best-effort defaults, while leaving more complex scenarios to user
configuration.

test/functional/options/shell_spec.lua

@@ -0,0 +1,78 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local eq = t.eq
+local is_os = t.is_os
+local skip = t.skip
+local api = n.api
+local clear = n.clear
+
+describe('applies sensible default options for different shells #28384', function()
+  ---@param sh string
+  ---@param shcf string
+  ---@param sp string
+  ---@param srr string
+  ---@param sxq string
+  ---@param ssl boolean
+  local function expect(sh, shcf, sp, srr, sxq, ssl)
+    local opt = { sh = sh, shcf = shcf, sp = sp, srr = srr, sxq = sxq, ssl = ssl }
+    for k, v in pairs(opt) do
+      eq(v, api.nvim_get_option_info2(k, {}).default)
+    end
+  end
+  it('cmd.exe', function()
+    skip(not is_os('win'), 'N/A: only works on Windows')
+    clear()
+    expect('cmd.exe', '/s /c', '2>&1| tee', '>%s 2>&1', '"', false)
+  end)
+
+  it('powershell(PowerShell 5.x)', function()
+    t.skip(not is_os('win'), 'N/A: only works on Windows')
+    clear {
+      env = { SHELL = 'powershell' },
+    }
+    expect('powershell', '-Command', '2>&1| tee', '>%s 2>&1', '', true)
+  end)
+
+  it('pwsh(PowerShell 7.x)', function()
+    clear {
+      env = { SHELL = 'pwsh' },
+    }
+    expect('pwsh', '-c', '2>&1| tee', '>%s 2>&1', '', true)
+  end)
+
+  it('csh', function()
+    clear {
+      env = { SHELL = 'csh' },
+    }
+    expect('csh', '-c', '|& tee', '>&', '', true)
+  end)
+
+  it('bash', function()
+    clear {
+      env = { SHELL = 'bash' },
+    }
+    expect('bash', '-c', '2>&1| tee', '>%s 2>&1', '', true)
+  end)
+
+  it('unknown', function()
+    clear {
+      env = { SHELL = 'unknown' },
+    }
+    expect(
+      'unknown',
+      '-c',
+      not is_os('win') and '| tee' or '2>&1| tee',
+      not is_os('win') and '>' or '>%s 2>&1',
+      '',
+      not is_os('win') and true or false
+    )
+  end)
+
+  it('even if the path contains spaces', function()
+    clear {
+      env = { SHELL = ('%s/foo bar/bash'):format(n.nvim_dir) },
+    }
+    expect(('"%s/foo bar/bash"'):format(n.nvim_dir), '-c', '2>&1| tee', '>%s 2>&1', '', true)
+  end)
+end)