On Windows, the macros COPY, REMOVE and REMOVE_DIR are allocated
strings, unlike on other platforms, so the way we use these returned
values, they are never freed.
Instead make win32_command() keep a copy of the allocated string as
static to the function and free it there at each next call (the returned
value being type-casted to (const gchar *)). It will still leak the last
string, but anyway gimptool is short-lived.
Also it should silence static analyzers.
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