diffcore-delta: avoid ignoring final 'line' of file

hash_chars() would hash lines to integers, and store them in a spanhash,
but cut lines at 64 characters.  Thus, whenever it reached 64 characters
or a newline, it would create a new spanhash.  The problem is, the final
part of the file might not end 64 characters after the previous 'line'
and might not end with a newline.  This could, for example, cause an
85-byte file with 12 lines and only the first character in the file
differing to appear merely 23% similar rather than the expected 97%.
Ensure the last line is included, and add a testcase that would have
caught this problem.

Signed-off-by: Elijah Newren <newren@gmail.com>

diffcore-delta.c

@@ -159,6 +159,10 @@ static struct spanhash_top *hash_chars(struct repository *r,
 		n = 0;
 		accum1 = accum2 = 0;
 	}
+	if (n > 0) {
+		hashval = (accum1 + accum2 * 0x61) % HASHBASE;
+		hash = add_spanhash(hash, hashval, n);
+	}
 	QSORT(hash->data, (size_t)1ul << hash->alloc_log2, spanhash_cmp);
 	return hash;
 }

t/t4001-diff-rename.sh

@@ -286,4 +286,28 @@ test_expect_success 'basename similarity vs best similarity' '
 	test_cmp expected actual
 '
 
+test_expect_success 'last line matters too' '
+	{
+		test_write_lines a 0 1 2 3 4 5 6 7 8 9 &&
+		printf "git ignores final up to 63 characters if not newline terminated"
+	} >no-final-lf &&
+	git add no-final-lf &&
+	git commit -m "original version of file with no final newline" &&
+
+	# Change ONLY the first character of the whole file
+	{
+		test_write_lines b 0 1 2 3 4 5 6 7 8 9 &&
+		printf "git ignores final up to 63 characters if not newline terminated"
+	} >no-final-lf &&
+	git add no-final-lf &&
+	git mv no-final-lf still-absent-final-lf &&
+	git commit -a -m "rename no-final-lf -> still-absent-final-lf" &&
+	git diff-tree -r -M --name-status HEAD^ HEAD >actual &&
+	sed -e "s/^R[0-9]*	/R	/" actual >actual.munged &&
+	cat >expected <<-\EOF &&
+	R	no-final-lf	still-absent-final-lf
+	EOF
+	test_cmp expected actual.munged
+'
+
 test_done