fixup! mingw: ensure valid CTYPE

Originally, the intention was to set `LC_CTYPE=C.UTF-8`, but that
elicited some bug report. Sadly, the to-be-fixed-up commit's message
does not record this properly. Having searched for a while, it would
appear to this developer that the issue was ConEmu, as mentioned here:

    https://groups.google.com/g/git-for-windows/c/9_abrdlqi48/m/S9pJE56ZDgAJ

This was a long time ago, and re-testing with a current ConEmu failed to
reproduce the original issue.

So let's just go ahead and set `LC_CTYPE=C.UTF-8` (unless something else
was _already_ specified via `LANG`, `LC_ALL` or `LC_CTYPE`).

This fixes https://github.com/git-for-windows/git/issues/2802

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

compat/mingw.c

@@ -3370,7 +3370,7 @@ static void setup_windows_environment(void)
 #endif
 
 	if (!getenv("LC_ALL") && !getenv("LC_CTYPE") && !getenv("LANG"))
-		setenv("LC_CTYPE", "C", 1);
+		setenv("LC_CTYPE", "C.UTF-8", 1);
 
 	/*
 	 * Change 'core.symlinks' default to false, unless native symlinks are