merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/Documentation/RelNotes/1.5.0.4.txt

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+GIT v1.5.0.4 Release Notes
+==========================
+
+Fixes since v1.5.0.3
+--------------------
+
+* Bugfixes
+
+  - git.el does not add duplicate sign-off lines.
+
+  - git-commit shows the full stat of the resulting commit, not
+    just about the files in the current directory, when run from
+    a subdirectory.
+
+  - "git-checkout -m '@{8 hours ago}'" had a funny failure from
+    eval; fixed.
+
+  - git-gui updates.
+
+* Documentation updates
+
+* User manual updates