Git bisect run should check for the existence of the script

[Jean Abou Samra <jean@abou-samra.fr>]

Hi,

When using 'git bisect run', if the name of the script is misspelled or 
if the script was not made executable, 'git bisect' considers it to be a 
failure and stops at the first revision after the one claimed good. It 
would be better in my eyes to error out.

$ git bisect start
$ git bisect bad HEAD
$ git bisect good HEAD~10
Bisecting: 4 revisions left to test after this (roughly 2 steps)
[344dce312a0cf86d5a5772d54843cc179acaf6e3] bpo-41228: Fix /a/are/ in 
monthcalendar() descripton (GH-21372)
$ git bisect run ./non-existent.sh
running ./non-existent.sh
/usr/lib/git-core/git-bisect: 247: ./non-existent.sh: not found
Bisecting: 2 revisions left to test after this (roughly 1 step)
[6fc732a2116e2c42b0431bb7e2a21719351af755] Fix typo in docs: 'created by 
th' -> 'created by the' (GH-21384)
running ./non-existent.sh
/usr/lib/git-core/git-bisect: 247: ./non-existent.sh: not found
Bisecting: 0 revisions left to test after this (roughly 0 steps)
[8182cc2e68a3c6ea5d5342fed3f1c76b0521fbc1] bpo-39573: Use the Py_TYPE() 
macro (GH-21433)
running ./non-existent.sh
/usr/lib/git-core/git-bisect: 247: ./non-existent.sh: not found
8182cc2e68a3c6ea5d5342fed3f1c76b0521fbc1 is the first bad commit
commit 8182cc2e68a3c6ea5d5342fed3f1c76b0521fbc1
Author: Victor Stinner <vstinner@python.org>
Date:   Fri Jul 10 12:40:38 2020 +0200

     bpo-39573: Use the Py_TYPE() macro (GH-21433)

     Replace obj->ob_type with Py_TYPE(obj).

  Modules/_elementtree.c       | 2 +-
  Objects/abstract.c           | 4 ++--
  Objects/genericaliasobject.c | 2 +-
  Objects/unicodeobject.c      | 4 ++--
  PC/_msi.c                    | 6 +++---
  PC/winreg.c                  | 4 ++--
  Tools/scripts/combinerefs.py | 2 +-
  7 files changed, 12 insertions(+), 12 deletions(-)
bisect run success


Best regards,
Jean Abou Samra