git-add: introduce --edit (to edit the diff vs. the index)

With "git add -e [<files>]", Git will fire up an editor with the current
diff relative to the index (i.e. what you would get with "git diff
[<files>]").

Now you can edit the patch as much as you like, including adding/removing
lines, editing the text, whatever.  Make sure, though, that the first
character of the hunk lines is still a space, a plus or a minus.

After you closed the editor, Git will adjust the line counts of the hunks
if necessary, thanks to the --recount option of apply, and commit the
patch.  Except if you deleted everything, in which case nothing happens
(for obvious reasons).

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

t/t3702-add-edit.sh

@@ -0,0 +1,109 @@
+#!/bin/sh
+#
+# Copyright (c) 2007 Johannes E. Schindelin
+#
+
+test_description='add -e basic tests'
+. ./test-lib.sh
+
+
+cat > file << EOF
+LO, praise of the prowess of people-kings
+of spear-armed Danes, in days long sped,
+we have heard, and what honor the athelings won!
+Oft Scyld the Scefing from squadroned foes,
+from many a tribe, the mead-bench tore,
+awing the earls. Since erst he lay
+friendless, a foundling, fate repaid him:
+for he waxed under welkin, in wealth he throve,
+till before him the folk, both far and near,
+who house by the whale-path, heard his mandate,
+gave him gifts:  a good king he!
+EOF
+
+test_expect_success 'setup' '
+
+	git add file &&
+	test_tick &&
+	git commit -m initial file
+
+'
+
+cat > expected-patch << EOF
+diff --git a/file b/file
+index b9834b5..0b8f197 100644
+--- a/file
++++ b/file
+@@ -1,11 +1,3 @@
+-LO, praise of the prowess of people-kings
+-of spear-armed Danes, in days long sped,
+-we have heard, and what honor the athelings won!
+-Oft Scyld the Scefing from squadroned foes,
+-from many a tribe, the mead-bench tore,
+-awing the earls. Since erst he lay
+-friendless, a foundling, fate repaid him:
+-for he waxed under welkin, in wealth he throve,
+-till before him the folk, both far and near,
+-who house by the whale-path, heard his mandate,
+-gave him gifts:  a good king he!
++#!$SHELL_PATH
++mv -f "\$1" orig-patch &&
++mv -f patch "\$1"
+EOF
+
+cat > patch << EOF
+diff --git a/file b/file
+index b9834b5..ef6e94c 100644
+--- a/file
++++ b/file
+@@ -3,1 +3,333 @@ of spear-armed Danes, in days long sped,
+ we have heard, and what honor the athelings won!
++
+ Oft Scyld the Scefing from squadroned foes,
+@@ -2,7 +1,5 @@ awing the earls. Since erst he lay
+ friendless, a foundling, fate repaid him:
++
+ for he waxed under welkin, in wealth he throve,
+EOF
+
+cat > expected << EOF
+diff --git a/file b/file
+index b9834b5..ef6e94c 100644
+--- a/file
++++ b/file
+@@ -1,10 +1,12 @@
+ LO, praise of the prowess of people-kings
+ of spear-armed Danes, in days long sped,
+ we have heard, and what honor the athelings won!
++
+ Oft Scyld the Scefing from squadroned foes,
+ from many a tribe, the mead-bench tore,
+ awing the earls. Since erst he lay
+ friendless, a foundling, fate repaid him:
++
+ for he waxed under welkin, in wealth he throve,
+ till before him the folk, both far and near,
+ who house by the whale-path, heard his mandate,
+EOF
+
+echo "#!$SHELL_PATH" >fake-editor.sh
+cat >> fake-editor.sh <<\EOF
+mv -f "$1" orig-patch &&
+mv -f patch "$1"
+EOF
+
+test_set_editor "$(pwd)/fake-editor.sh"
+chmod a+x fake-editor.sh
+
+test_expect_success 'add -e' '
+
+	cp fake-editor.sh file &&
+	git add -e &&
+	test_cmp fake-editor.sh file &&
+	test_cmp orig-patch expected-patch &&
+	git diff --cached > out &&
+	test_cmp out expected
+
+'
+
+test_done