bug#55427: emacs-libgit tests are broken

[Liliana Marie Prikler <liliana.prikler@gmail.com>]

With the update to Emacs 28, emacs-libgit (a requirement for emacs-
magit) fails to build with the following test error:

Test project /tmp/guix-build-emacs-libgit-20200515-1.0ef8b13.drv-
0/build
      Start  1: libegit2_annotated-commit
 1/29 Test  #1: libegit2_annotated-commit ........   Passed    0.38 sec
      Start  2: libegit2_blame
 2/29 Test  #2: libegit2_blame ...................   Passed    0.39 sec
      Start  3: libegit2_blob
 3/29 Test  #3: libegit2_blob ....................***Failed    0.43 sec
Running 4 tests (2022-05-15 07:25:56+0000, selector ‘t’)
Test blob-binary backtrace:
  signal(wrong-type-argument (utf-8-string-p "\177ELF\2\1\1\0\0\0\0\0\
  apply(signal (wrong-type-argument (utf-8-string-p "\177ELF\2\1\1\0\0
  (setq value-42 (apply fn-40 args-41))
  (unwind-protect (setq value-42 (apply fn-40 args-41)) (setq form-des
  (if (unwind-protect (setq value-42 (apply fn-40 args-41)) (setq form
  (let (form-description-44) (if (unwind-protect (setq value-42 (apply
  (let ((value-42 'ert-form-evaluation-aborted-43)) (let (form-descrip
  (let* ((fn-40 #'string=) (args-41 (condition-case err (let ((signal-
  (let* ((repo (libgit-repository-open path)) (blob (libgit-revparse-s
  (let ((default-directory path)) (init) (commit-change "filename" str
  (progn (make-directory path 'parents) (let ((default-directory path)
  (unwind-protect (progn (make-directory path 'parents) (let ((default
  (let ((path "/tmp/guix-build-emacs-libgit-20200515-1.0ef8b13.dr...")
  (let* ((str (unibyte-string 127 69 76 70 2 1 1 0 0 0 0 0 0 0 0 0 3 0
  (let ((lexical-binding nil)) (let* ((str (unibyte-string 127 69 76 7
  (lambda nil (let ((lexical-binding nil)) (let* ((str (unibyte-string
  ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
  ert-run-test(#s(ert-test :name blob-binary :documentation nil :body 
  ert-run-or-rerun-test(#s(ert--stats :selector t :tests [#s(ert-test 
  ert-run-tests(t #f(compiled-function (event-type &rest event-args) #
  ert-run-tests-batch(nil)
  ert-run-tests-batch-and-exit()
  command-line-1(("-L" "/tmp/guix-build-emacs-libgit-20200515-1.0ef8b1
  command-line()
  normal-top-level()
Test blob-binary condition:
    (wrong-type-argument utf-8-string-p
"\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260\7\0\0\0\0\0\0@\0\0\
0\0\0\0\0\270\33\0\0\0\0\0\0\0\0\0\0@\08\0\11\0@\0\35\0\34\0\6\0\0\0\5\
0\0\0@\0\0\0\0\0\0\0")
   FAILED  1/4  blob-binary (0.041795 sec)
   passed  2/4  blob-create-fromdisk (0.023014 sec)
   passed  3/4  blob-create-fromstring (0.022546 sec)
   passed  4/4  blob-text (0.044085 sec)

Ran 4 tests, 3 results as expected, 1 unexpected (2022-05-15
07:25:56+0000, 0.301178 sec)

1 unexpected results:
   FAILED  blob-binary
[...]

I have no idea what causes it or where utf-8-string-p even comes from,
but the implementation of string= appears to have changed.  When
running the build locally another test also appears flaky due to weird
time stamp conversions.

Cheers