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.
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