abspath_part_inside_repo: respect core.ignoreCase

If the file system is case-insensitive, we really must be careful to
ignore differences in case only.

This fixes https://github.com/git-for-windows/git/issues/735

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

t/t3700-add.sh

@@ -402,4 +402,11 @@ test_expect_success 'all statuses changed in folder if . is given' '
 	test $(git ls-files --stage | grep ^100755 | wc -l) -eq 0
 '
 
+test_expect_success CASE_INSENSITIVE_FS 'path is case-insensitive' '
+	path="$(pwd)/BLUB" &&
+	touch "$path" &&
+	downcased="$(echo "$path" | tr A-Z a-z)" &&
+	git add "$downcased"
+'
+
 test_done