diff: expose the hunk header

We can rebuild it from the data we already have, but this will allow
us to get the function headers in it once the library supports it.

include/pygit2/types.h

@@ -68,6 +68,7 @@ typedef struct {
 
 typedef struct {
     PyObject_HEAD
+    char *header;
     int old_start;
     int old_lines;
     char* old_file;

src/pygit2/diff.c

@@ -101,6 +101,16 @@ static int diff_hunk_cb(
     hunk->new_start = range->new_start;
     hunk->new_lines = range->new_lines;
 
+    if (header) {
+        hunk->header = malloc(header_len+1);
+
+        if (hunk->header == NULL)
+            return -1;
+
+        memcpy(hunk->header, header, header_len);
+        hunk->header[header_len] = '\0';
+    }
+
     if (delta->old_file.path != NULL) {
         len = strlen(delta->old_file.path) + 1;
         old_path = malloc(sizeof(char) * len);
@@ -219,6 +229,7 @@ Hunk_dealloc(Hunk *self)
 }
 
 PyMemberDef Hunk_members[] = {
+    {"header",    T_STRING, offsetof(Hunk, header), 0, "header"},
     {"old_start", T_INT, offsetof(Hunk, old_start), 0, "old start"},
     {"old_lines", T_INT, offsetof(Hunk, old_lines), 0, "old lines"},
     {"old_file",  T_STRING, offsetof(Hunk, old_file), 0, "old file"},

test/test_diff.py

@@ -174,6 +174,12 @@ class DiffTest(utils.BareRepoTestCase):
         diff = commit_a.tree.diff(commit_b.tree)
         self.assertEqual(diff.patch, PATCH)
 
+    def test_diff_header(self):
+        commit_a = self.repo[COMMIT_SHA1_1]
+        commit_b = self.repo[COMMIT_SHA1_2]
+        diff = commit_a.tree.diff(commit_b.tree)
+
+        self.assertEqual(diff.changes['hunks'][0].header, "@@ -1 +1 @@\n")
 
 if __name__ == '__main__':
     unittest.main()