rebase: fix garbled progress display with '-x'

rebase: fix garbled progress display with '-x'

When running a command with the 'exec' instruction during an
interactive rebase session, or for a range of commits using 'git
rebase -x', the output can be a bit garbled when the name of the
command is short enough:

  $ git rebase -x true HEAD~5
  Executing: true
  Executing: true
  Executing: true
  Executing: true
  Executing: true)
  Successfully rebased and updated refs/heads/master.

Note the ')' at the end of the last line.  It gets more garbled as the
range of commits increases:

  $ git rebase -x true HEAD~50
  Executing: true)
  [ repeated 3 more times ]
  Executing: true0)
  [ repeated 44 more times ]
  Executing: true00)
  Successfully rebased and updated refs/heads/master.

Those extra numbers and ')' are remnants of the previously displayed
"Rebasing (N/M)" progress lines that are usually completely
overwritten by the "Executing: <cmd>" lines, unless 'cmd' is short and
the "N/M" part is long.

Make sure that the previously displayed "Rebasing (N/M)" line is
cleared by using the term_clear_line() helper function added in the
previous patch.  Do so only when not being '--verbose', because in
that case these "Rebasing (N/M)" lines are not printed as progress
(i.e. as lines with '\r' at the end), but as "regular" output (with
'\n' at the end).

A couple of other rebase commands print similar messages, e.g.
"Stopped at <abbrev-oid>... <subject>" for the 'edit' or 'break'
commands, or the "Successfully rebased and updated <full-ref>." at the
very end.  These are so long that they practically always overwrite
that "Rebasing (N/M)" progress line, but let's be prudent, and clear
the last line before printing these, too.

In 't3420-rebase-autostash.sh' two helper functions prepare the
expected output of four tests that check the full output of 'git
rebase' and thus are affected by this change, so adjust their
expectations to account for the new line clearing.

Note that this patch doesn't completely eliminate the possibility of
similar garbled outputs, e.g. some error messages from rebase or the
"Auto-merging <file>" message from within the depths of the merge
machinery might not be long enough to completely cover the last
"Rebasing (N/M)" line.  This patch doesn't do anything about them,
because dealing with them individually would result in way too much
churn, while having a catch-all term_clear_line() call in the common
code path of pick_commits() would hide the "Rebasing (N/M)" line way
too soon, and it would either flicker or be invisible.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>