mingw: demonstrate a git add issue with NTFS junctions

NTFS junctions are somewhat similar in spirit to Unix bind mounts: they
point to a different directory and are resolved by the filesystem
driver. As such, they appear to `lstat()` as if they are directories,
not as if they are symbolic links.

_Any_ user can create junctions, while symbolic links can only be
created by non-administrators in Developer Mode on Windows 10. Hence
NTFS junctions are much more common "in the wild" than NTFS symbolic
links.

It was reported in #2481
that adding files via an absolute path that traverses an NTFS junction:
since 1e64d18 (mingw: do resolve symlinks in `getcwd()`), we resolve not
only symbolic links but also NTFS junctions when determining the
absolute path of the current directory. The same is not true for `git
add <file>`, where symbolic links are resolved in `<file>`, but not NTFS
junctions.

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

t/t3700-add.sh

@@ -549,4 +549,15 @@ test_expect_success CASE_INSENSITIVE_FS 'path is case-insensitive' '
 	git add "$downcased"
 '
 
+test_expect_failure MINGW 'can add files via NTFS junctions' '
+	test_when_finished "cmd //c rmdir junction && rm -rf target" &&
+	test_create_repo target &&
+	cmd //c "mklink /j junction target" &&
+	>target/via-junction &&
+	git -C junction add "$(pwd)/junction/via-junction" &&
+	echo via-junction >expect &&
+	git -C target diff --cached --name-only >actual &&
+	test_cmp expect actual
+'
+
 test_done