add--interactive: handle initial commit better

There were several points where we looked at the HEAD
commit; for initial commits, this is meaningless. So instead
we:

  - show staged status data as a diff against the empty tree
    instead of HEAD
  - show file diffs as creation events
  - use "git rm --cached" to revert instead of going back to
    the HEAD commit

We magically reference the empty tree to implement this.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3701-add-interactive.sh

@@ -0,0 +1,69 @@
+#!/bin/sh
+
+test_description='add -i basic tests'
+. ./test-lib.sh
+
+test_expect_success 'setup (initial)' '
+	echo content >file &&
+	git add file &&
+	echo more >>file &&
+	echo lines >>file
+'
+test_expect_success 'status works (initial)' '
+	git add -i </dev/null >output &&
+	grep "+1/-0 *+2/-0 file" output
+'
+cat >expected <<EOF
+new file mode 100644
+index 0000000..d95f3ad
+--- /dev/null
++++ b/file
+@@ -0,0 +1 @@
++content
+EOF
+test_expect_success 'diff works (initial)' '
+	(echo d; echo 1) | git add -i >output &&
+	sed -ne "/new file/,/content/p" <output >diff &&
+	diff -u expected diff
+'
+test_expect_success 'revert works (initial)' '
+	git add file &&
+	(echo r; echo 1) | git add -i &&
+	git ls-files >output &&
+	! grep . output
+'
+
+test_expect_success 'setup (commit)' '
+	echo baseline >file &&
+	git add file &&
+	git commit -m commit &&
+	echo content >>file &&
+	git add file &&
+	echo more >>file &&
+	echo lines >>file
+'
+test_expect_success 'status works (commit)' '
+	git add -i </dev/null >output &&
+	grep "+1/-0 *+2/-0 file" output
+'
+cat >expected <<EOF
+index 180b47c..b6f2c08 100644
+--- a/file
++++ b/file
+@@ -1 +1,2 @@
+ baseline
++content
+EOF
+test_expect_success 'diff works (commit)' '
+	(echo d; echo 1) | git add -i >output &&
+	sed -ne "/^index/,/content/p" <output >diff &&
+	diff -u expected diff
+'
+test_expect_success 'revert works (commit)' '
+	git add file &&
+	(echo r; echo 1) | git add -i &&
+	git add -i </dev/null >output &&
+	grep "unchanged *+3/-0 file" output
+'
+
+test_done