t9300: document fast-import empty path issues

When given an empty path, fast-import sometimes reports "missing"
instead of using the root tree object.  On top of this, for "ls" and
file copy (but not move) it dies with "Empty path component found in
input".

Document this behaviour with failing test cases.

Reported-by: Dave Abrahams <dave@boostpro.com>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9300-fast-import.sh

@@ -1031,6 +1031,32 @@ test_expect_success \
 	 git diff-tree -M -r M3^ M3 >actual &&
 	 compare_diff_raw expect actual'
 
+cat >input <<INPUT_END
+commit refs/heads/M4
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+rename root
+COMMIT
+
+from refs/heads/M2^0
+R "" sub
+
+INPUT_END
+
+cat >expect <<EOF
+:100644 100644 7123f7f44e39be127c5eb701e5968176ee9d78b1 7123f7f44e39be127c5eb701e5968176ee9d78b1 R100	file2/oldf	sub/file2/oldf
+:100755 100755 85df50785d62d3b05ab03d9cbf7e4a0b49449730 85df50785d62d3b05ab03d9cbf7e4a0b49449730 R100	file4	sub/file4
+:100755 100755 f1fb5da718392694d0076d677d6d0e364c79b0bc f1fb5da718392694d0076d677d6d0e364c79b0bc R100	i/am/new/to/you	sub/i/am/new/to/you
+:100755 100755 e74b7d465e52746be2b4bae983670711e6e66657 e74b7d465e52746be2b4bae983670711e6e66657 R100	newdir/exec.sh	sub/newdir/exec.sh
+:100644 100644 fcf778cda181eaa1cbc9e9ce3a2e15ee9f9fe791 fcf778cda181eaa1cbc9e9ce3a2e15ee9f9fe791 R100	newdir/interesting	sub/newdir/interesting
+EOF
+test_expect_failure \
+	'M: rename root to subdirectory' \
+	'git fast-import <input &&
+	 git diff-tree -M -r M4^ M4 >actual &&
+	 cat actual &&
+	 compare_diff_raw expect actual'
+
 ###
 ### series N
 ###
@@ -1227,6 +1253,29 @@ test_expect_success \
 	 git diff-tree -C --find-copies-harder -r N4 N6 >actual &&
 	 compare_diff_raw expect actual'
 
+test_expect_failure \
+	'N: copy root by path' \
+	'cat >expect <<-\EOF &&
+	:100755 100755 f1fb5da718392694d0076d677d6d0e364c79b0bc f1fb5da718392694d0076d677d6d0e364c79b0bc C100	file2/newf	oldroot/file2/newf
+	:100644 100644 7123f7f44e39be127c5eb701e5968176ee9d78b1 7123f7f44e39be127c5eb701e5968176ee9d78b1 C100	file2/oldf	oldroot/file2/oldf
+	:100755 100755 85df50785d62d3b05ab03d9cbf7e4a0b49449730 85df50785d62d3b05ab03d9cbf7e4a0b49449730 C100	file4	oldroot/file4
+	:100755 100755 e74b7d465e52746be2b4bae983670711e6e66657 e74b7d465e52746be2b4bae983670711e6e66657 C100	newdir/exec.sh	oldroot/newdir/exec.sh
+	:100644 100644 fcf778cda181eaa1cbc9e9ce3a2e15ee9f9fe791 fcf778cda181eaa1cbc9e9ce3a2e15ee9f9fe791 C100	newdir/interesting	oldroot/newdir/interesting
+	EOF
+	 cat >input <<-INPUT_END &&
+	commit refs/heads/N-copy-root-path
+	committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data <<COMMIT
+	copy root directory by (empty) path
+	COMMIT
+
+	from refs/heads/branch^0
+	C "" oldroot
+	INPUT_END
+	 git fast-import <input &&
+	 git diff-tree -C --find-copies-harder -r branch N-copy-root-path >actual &&
+	 compare_diff_raw expect actual'
+
 test_expect_success \
 	'N: delete directory by copying' \
 	'cat >expect <<-\EOF &&
@@ -2934,4 +2983,20 @@ test_expect_success 'S: ls with garbage after sha1 must fail' '
 	test_i18ngrep "space after tree-ish" err
 '
 
+###
+### series T (ls)
+###
+# Setup is carried over from series S.
+
+test_expect_failure 'T: ls root tree' '
+	sed -e "s/Z\$//" >expect <<-EOF &&
+	040000 tree $(git rev-parse S^{tree})	Z
+	EOF
+	sha1=$(git rev-parse --verify S) &&
+	git fast-import --import-marks=marks <<-EOF >actual &&
+	ls $sha1 ""
+	EOF
+	test_cmp expect actual
+'
+
 test_done