mingw: ensure valid CTYPE

A change between versions 2.4.1 and 2.6.0 of the MSYS2 runtime modified
how Cygwin's runtime (and hence Git for Windows' MSYS2 runtime
derivative) handles locales: d16a56306d (Consolidate wctomb/mbtowc calls
for POSIX-1.2008, 2016-07-20).

An unintended side-effect is that "cold-calling" into the POSIX
emulation will start with a locale based on the current code page,
something that Git for Windows is very ill-prepared for, as it expects
to be able to pass a command-line containing non-ASCII characters to the
shell without having those characters munged.

One symptom of this behavior: when `git clone` or `git fetch` shell out
to call `git-upload-pack` with a path that contains non-ASCII
characters, the shell tried to interpret the entire command-line
(including command-line parameters) as executable path, which obviously
must fail.

This fixes git-for-windows#1036

Signed-off-by: Johannes Schindelin <[email protected]>

compat/mingw.c

@@ -2913,6 +2913,9 @@ static void setup_windows_environment(void)
 	 */
 	if (!(tmp = getenv("MSYS")) || !strstr(tmp, "winsymlinks:nativestrict"))
 		has_symlinks = 0;
+
+	if (!getenv("LC_ALL") && !getenv("LC_CTYPE") && !getenv("LANG"))
+		setenv("LC_CTYPE", "C", 1);
 }
 
 int handle_long_path(wchar_t *path, int len, int max_path, int expand)