Protect get_author_ident_from_commit() from filenames in work tree

We used to use "cat-file commit $commit" to extract the original
author information from existing commit, but an earlier commit
5ac2715 (Consistent message encoding while reusing log from an
existing commit) changed it to use "git show -s $commit".  If
you have a file in your work tree that can be interpreted as a
valid object name (e.g. "HEAD"), this conversion will not work.

Disambiguate by marking the end of revision parameter on the
comand line with an explicit "--" to fix this.

This breakage is most visible with rebase when a file called
"HEAD" exists in the worktree.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-sh-setup.sh

@@ -119,7 +119,7 @@ get_author_ident_from_commit () {
 	}
 	'
 	encoding=$(git config i18n.commitencoding || echo UTF-8)
-	git show -s --pretty=raw --encoding="$encoding" "$1" |
+	git show -s --pretty=raw --encoding="$encoding" "$1" -- |
 	LANG=C LC_ALL=C sed -ne "$pick_author_script"
 }
 

t/t3404-rebase-interactive.sh

@@ -340,4 +340,26 @@ test_expect_success 'rebase a commit violating pre-commit' '
 
 '
 
+test_expect_success 'rebase with a file named HEAD in worktree' '
+
+	rm -fr .git/hooks &&
+	git reset --hard &&
+	git checkout -b branch3 A &&
+
+	(
+		GIT_AUTHOR_NAME="Squashed Away" &&
+		export GIT_AUTHOR_NAME &&
+		>HEAD &&
+		git add HEAD &&
+		git commit -m "Add head" &&
+		>BODY &&
+		git add BODY &&
+		git commit -m "Add body"
+	) &&
+
+	FAKE_LINES="1 squash 2" git rebase -i to-be-rebased &&
+	test "$(git show -s --pretty=format:%an)" = "Squashed Away"
+
+'
+
 test_done