rebase -i: squash should retain the authorship of the _first_ commit

It was determined on the mailing list, that it makes more sense for a
"squash" to keep the author of the first commit as the author for the
result of the squash.

Make it so.

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

Documentation/git-rebase.txt

@@ -293,7 +293,7 @@ rebasing.
 If you want to fold two or more commits into one, replace the command
 "pick" with "squash" for the second and subsequent commit.  If the
 commits had different authors, it will attribute the squashed commit to
-the author of the last commit.
+the author of the first commit.
 
 In both cases, or when a "pick" does not succeed (because of merge
 errors), the loop will stop to let you fix things, and you can continue

git-rebase--interactive.sh

@@ -261,9 +261,9 @@ do_next () {
 		esac
 
 		failed=f
+		author_script=$(get_author_ident_from_commit HEAD)
 		output git reset --soft HEAD^
 		pick_one -n $sha1 || failed=t
-		author_script=$(get_author_ident_from_commit $sha1)
 		echo "$author_script" > "$DOTEST"/author-script
 		case $failed in
 		f)

t/t3404-rebase-interactive.sh

@@ -180,7 +180,7 @@ test_expect_success 'squash' '
 '
 
 test_expect_success 'retain authorship when squashing' '
-	git show HEAD | grep "^Author: Nitfol"
+	git show HEAD | grep "^Author: Twerp Snog"
 '
 
 test_expect_success 'preserve merges with -p' '