fixup??? mingw: allow git.exe to be used instead of the "Git wrapper"

This patch makes it so that Git really sets the desired `MSYSTEM` on
Windows/ARM64.

The reason this did not work (and my faulty tests did not catch that
before merging #5586) is that
clang (at least version 20.1.3 as built from
https://github.com/git-for-windows/MINGW-packages
8df0c2fff4184deff15acce9bfd791fb6e0d60fe) predefines the `__MINGW64__`
constant:

	$ echo | clang -dM -E - | grep -n MINGW64
	249:#define __MINGW64__ 1

Let's just switch the order between the CLANGARM64 and the MINGW64 test;
This will still work for MINGW64 because none of the constants used in
the CLANGARM64 condition are defined for x64 gcc (or for that matter,
clang).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

compat/mingw.c

@@ -3746,10 +3746,10 @@ static void setup_windows_environment(void)
 		char buf[32768];
 		size_t off = 0;
 
-#if defined(__MINGW64__) || defined(_M_AMD64)
-		setenv("MSYSTEM", "MINGW64", 1);
-#elif defined(__aarch64__) || defined(_M_ARM64) || defined(_M_ARM64EC)
+#if defined(__aarch64__) || defined(_M_ARM64) || defined(_M_ARM64EC)
 		setenv("MSYSTEM", "CLANGARM64", 1);
+#elif defined(__MINGW64__) || defined(_M_AMD64)
+		setenv("MSYSTEM", "MINGW64", 1);
 #else
 		setenv("MSYSTEM", "MINGW32", 1);
 #endif