Add git_diff_patch_print

This adds a `git_diff_patch_print()` API which is more like the
existing API to "print" a patch from an entire `git_diff_list`
but operates on a single `git_diff_patch` object.

Also, it rewrites the `git_diff_patch_to_str()` API to use that
function (making it very small).

include/git2/diff.h

@@ -604,10 +604,27 @@ GIT_EXTERN(int) git_diff_patch_get_line_in_hunk(
 	size_t line_of_hunk);
 
 /**
+ * Serialize the patch to text via callback.
+ *
+ * Returning a non-zero value from the callback will terminate the iteration
+ * and cause this return `GIT_EUSER`.
+ *
+ * @param patch A git_diff_patch representing changes to one file
+ * @param cb_data Reference pointer that will be passed to your callbacks.
+ * @param print_cb Callback function to output lines of the patch.  Will be
+ *                 called for file headers, hunk headers, and diff lines.
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
+ */
+GIT_EXTERN(int) git_diff_patch_print(
+	git_diff_patch *patch,
+	void *cb_data,
+	git_diff_data_fn print_cb);
+
+/**
  * Get the content of a patch as a single diff text.
  *
  * @param string Allocated string; caller must free.
- * @param patch The patch to generate a string from.
+ * @param patch A git_diff_patch representing changes to one file
  * @return 0 on success, <0 on failure.
  */
 GIT_EXTERN(int) git_diff_patch_to_str(

src/diff_output.c

@@ -1511,26 +1511,25 @@ static int print_to_buffer_cb(
     size_t content_len)
 {
 	git_buf *output = cb_data;
-	GIT_UNUSED(delta);
-	GIT_UNUSED(range);
-	GIT_UNUSED(line_origin);
+	GIT_UNUSED(delta); GIT_UNUSED(range); GIT_UNUSED(line_origin);
 	return git_buf_put(output, content, content_len);
 }
 
-int git_diff_patch_to_str(
-	char **string,
-	git_diff_patch *patch)
+int git_diff_patch_print(
+	git_diff_patch *patch,
+	void *cb_data,
+	git_diff_data_fn print_cb)
 {
 	int error;
-	git_buf output = GIT_BUF_INIT, temp = GIT_BUF_INIT;
+	git_buf temp = GIT_BUF_INIT;
 	diff_print_info pi;
 	size_t h, l;
 
-	assert(string && patch);
+	assert(patch && print_cb);
 
 	pi.diff     = patch->diff;
-	pi.print_cb = print_to_buffer_cb;
-	pi.cb_data  = &output;
+	pi.print_cb = print_cb;
+	pi.cb_data  = cb_data;
 	pi.buf      = &temp;
 
 	error = print_patch_file(&pi, patch->delta, 0);
@@ -1550,16 +1549,27 @@ int git_diff_patch_to_str(
 		}
 	}
 
+	git_buf_free(&temp);
+
+	return error;
+}
+
+int git_diff_patch_to_str(
+	char **string,
+	git_diff_patch *patch)
+{
+	int error;
+	git_buf output = GIT_BUF_INIT;
+
+	error = git_diff_patch_print(patch, &output, print_to_buffer_cb);
+
 	/* GIT_EUSER means git_buf_put in print_to_buffer_cb returned -1,
 	 * meaning a memory allocation failure, so just map to -1...
 	 */
 	if (error == GIT_EUSER)
 		error = -1;
 
-	git_buf_free(&temp);
-
 	*string = git_buf_detach(&output);
 
 	return error;
 }
-