git commit --amend --no-edit rewrote the commit and moved the branch tip
even when nothing changed, because the committer date was reset to "now".
Reuse the existing committer date so a no-op amend keeps the commit hash and
leaves the branch untouched.

A real change (tree, message, author, committer, or signing) still rewrites
as before.

Harald Nordgren (2):
  commit: extract commit_index_files_or_die() helper
  commit: keep the commit on a no-op amend

 Documentation/git-commit.adoc         |   6 ++
 builtin/commit.c                      |  69 ++++++++++++++-
 t/t7501-commit-basic-functionality.sh | 119 ++++++++++++++++++++++++++
 3 files changed, 190 insertions(+), 4 deletions(-)

base-commit: ea97ad8d017de0c9037451a78008a0fd60abea0c

Submitted-As: https://lore.kernel.org/git/pull.2334.git.git.1781342189.gitgitgadget@gmail.com