mingw: special-case `open(symlink, O_CREAT | O_EXCL)`

The `_wopen()` function would gladly follow a symbolic link to a
non-existent file and create it when given above-mentioned flags.

Git expects the `open()` call to fail, though. So let's add yet another
work-around to pretend that Windows behaves according to POSIX, see:
https://pubs.opengroup.org/onlinepubs/007904875/functions/open.html#:~:text=If%20O_CREAT%20and%20O_EXCL%20are,set%2C%20the%20result%20is%20undefined.

This is required to let t4115.8(--reject removes .rej symlink if it
exists) pass on Windows when enabling the MSYS2 runtime's symbolic link
support.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

diff --git a/compat/mingw.c b/compat/mingw.c
index 90ba5ce..ba1b7b6 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -629,6 +629,7 @@ int mingw_open (const char *filename, int oflags, ...)
 	int fd, create = (oflags & (O_CREAT | O_EXCL)) == (O_CREAT | O_EXCL);
 	wchar_t wfilename[MAX_PATH];
 	open_fn_t open_fn;
+	WIN32_FILE_ATTRIBUTE_DATA fdata;
 
 	DECLARE_PROC_ADDR(ntdll.dll, NTSTATUS, NTAPI, RtlGetLastNtStatus, void);
 
@@ -653,6 +654,19 @@ int mingw_open (const char *filename, int oflags, ...)
 	else if (xutftowcs_path(wfilename, filename) < 0)
 		return -1;
 
+	/*
+	 * When `symlink` exists and is a symbolic link pointing to a
+	 * non-existing file, `_wopen(symlink, O_CREAT | O_EXCL)` would
+	 * create that file. Not what we want: Linux would say `EEXIST`
+	 * in that instance, which is therefore what Git expects.
+	 */
+	if (create &&
+	    GetFileAttributesExW(wfilename, GetFileExInfoStandard, &fdata) &&
+	    (fdata.dwFileAttributes & FILE_ATTRIBUTE_REPARSE_POINT)) {
+		errno = EEXIST;
+		return -1;
+	}
+
 	fd = open_fn(wfilename, oflags, mode);
 
 	/*