rebase -i: respect core.abbrev

collapse_todo_ids() uses `git rev-parse --short=7' to abbreviate
commit ids before showing them to the user in a text editor.  Let's
drop argument from --short to the configured value instead (still
defaulting to 7).

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

git-rebase--interactive.sh

@@ -713,7 +713,7 @@ expand_todo_ids() {
 }
 
 collapse_todo_ids() {
-	transform_todo_ids --short=7
+	transform_todo_ids --short
 }
 
 # Rearrange the todo list that has both "pick sha1 msg" and