rebase -i: use full object name internally throughout the script

rebase -i: use full object name internally throughout the script

In earlier days, the abbreviated commit object name shown to the end
users were generated with hardcoded --abbrev=7; 56895038 (rebase
-i: respect core.abbrev, 2013-09-28) tried to make it honor the user
specified core.abbrev, but it missed the very initial invocation of
the editor.

These days, we try to use the full 40-hex object names internally to
avoid ambiguity that can arise after rebase starts running.  Newly
created objects during the rebase may share the same prefix with
existing commits listed in the insn sheet.  These object names are
shortened just before invoking the sequence editor to present the
insn sheet to the end user, and then expanded back to full object
names when the editor returns.

But the code still used the shortened names when preparing the insn
sheet for the very first time, resulting "7 hexdigits or more"
output to the user.  Change the code to use full 40-hex commit
object names from the very beginning to make things more uniform.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>