am --abort: keep unrelated commits on unborn branch

Since 7b3b7e37 (am --abort: keep unrelated commits since the last failure
and warn, 2010-12-21), git-am would refuse to rewind HEAD if commits
were made since the last git-am failure. This check was implemented in
safe_to_abort(), which checked to see if HEAD's hash matched the
abort-safety file.

However, this check was skipped if the abort-safety file was empty,
which can happen if git-am failed while on an unborn branch. As such, if
any commits were made since then, they would be discarded. Fix this by
carrying on the abort safety check even if the abort-safety file is
empty.
Signed-off-by: NPaul Tan <pyokagan@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-am.sh

@@ -86,7 +86,7 @@ safe_to_abort () {
 		return 1
 	fi
 
-	if ! test -s "$dotest/abort-safety"
+	if ! test -f "$dotest/abort-safety"
 	then
 		return 0
 	fi

t/t4151-am-abort.sh

@@ -142,4 +142,15 @@ test_expect_success 'am -3 --abort on unborn branch removes applied commits' '
 	test refs/heads/orphan = "$(git symbolic-ref HEAD)"
 '
 
+test_expect_success 'am --abort on unborn branch will keep local commits intact' '
+	git checkout -f --orphan orphan &&
+	git reset &&
+	test_must_fail git am 0004-*.patch &&
+	test_commit unrelated2 &&
+	git rev-parse HEAD >expect &&
+	git am --abort &&
+	git rev-parse HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_done