files: extract pre-processing part from merge()

I'll make the first half generic over T: AsRef<[u8]>.

lib/src/files.rs

@@ -162,8 +162,10 @@ pub fn merge(slices: &Merge<&[u8]>) -> MergeResult {
     // more than 3 parts?
     let num_diffs = slices.removes().len();
     let diff_inputs = slices.removes().chain(slices.adds());
+    merge_hunks(&Diff::by_line(diff_inputs), num_diffs)
+}
 
-    let diff = Diff::by_line(diff_inputs);
+fn merge_hunks(diff: &Diff, num_diffs: usize) -> MergeResult {
     let mut resolved_hunk = ContentHunk(vec![]);
     let mut merge_hunks: Vec<Merge<ContentHunk>> = vec![];
     for diff_hunk in diff.hunks() {