rebase -i: use a better reflog message

Now that the "checkout" invoked internally from "rebase -i" knows to
honor GIT_REFLOG_ACTION, we can start to use it to write a better
reflog message when "rebase anotherbranch", "rebase --onto branch",
etc. internally checks out the new fork point.  We will write:

  rebase -i: checkout master

instead of the old

  rebase -i

As all the calls git-rebase--interactive make to underlying git
commands that leave reflog messages are preceded by the internal
comment_for_reflog helper function, which uses the original value of
the GIT_REFLOG_ACTION variable it saw when it first started, the new
assignments to GIT_REFLOG_ACTION actively contaminate the value of
the variable, knowing that it will be reset to a sane value before
it is used again.  This does not generally hold true but it should
suffice for now.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -837,8 +837,11 @@ comment_for_reflog start
 
 if test ! -z "$switch_to"
 then
+	GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $switch_to"
 	output git checkout "$switch_to" -- ||
-		die "Could not checkout $switch_to"
+	die "Could not checkout $switch_to"
+
+	comment_for_reflog start
 fi
 
 orig_head=$(git rev-parse --verify HEAD) || die "No HEAD?"
@@ -980,6 +983,7 @@ has_action "$todo" ||
 
 test -d "$rewritten" || test -n "$force_rebase" || skip_unnecessary_picks
 
+GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $onto_name"
 output git checkout $onto || die_abort "could not detach HEAD"
 git update-ref ORIG_HEAD $orig_head
 do_rest

t/t3404-rebase-interactive.sh

@@ -934,6 +934,21 @@ test_expect_success 'rebase --edit-todo can be used to modify todo' '
 	test L = $(git cat-file commit HEAD | sed -ne \$p)
 '
 
+test_expect_success 'rebase -i produces readable reflog' '
+	git reset --hard &&
+	git branch -f branch-reflog-test H &&
+	git rebase -i --onto I F branch-reflog-test &&
+	cat >expect <<-\EOF &&
+	rebase -i (start): checkout I
+	rebase -i (pick): G
+	rebase -i (pick): H
+	rebase -i (finish): returning to refs/heads/branch-reflog-test
+	EOF
+	tail -n 4 .git/logs/HEAD |
+	sed -e "s/.*	//" >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'rebase -i respects core.commentchar' '
 	git reset --hard &&
 	git checkout E^0 &&