Expose patch serialization to git_buf

Returning library-allocated strings from libgit2 works fine on Linux,
but may cause problems on Windows because there is no one C Runtime that
everything links against. With libgit2 not exposing its own allocator,
freeing the string is a gamble.

git_patch_to_str already serializes to a buffer, then returns the
underlying memory. Expose the functionality directly, so callers can use
the git_buf_free function to free the memory later.

include/git2/patch.h

@@ -242,6 +242,16 @@ GIT_EXTERN(int) git_patch_to_str(
 	char **string,
 	git_patch *patch);
 
+/**
+ * Get the content of a patch as a single diff text.
+ *
+ * @param out The git_buf to be filled in
+ * @param patch A git_patch representing changes to one file
+ * @return 0 on success, <0 on failure.
+ */
+GIT_EXTERN(int) git_patch_to_buf(
+	git_buf *out,
+	git_patch *patch);
 
 GIT_END_DECL
 

src/diff_print.c

@@ -452,7 +452,15 @@ static int diff_print_to_buffer_cb(
 	return git_buf_put(output, line->content, line->content_len);
 }
 
-/* print a git_patch to a string buffer */
+/* print a git_patch to a git_buf */
+int git_patch_to_buf(
+	git_buf *out,
+	git_patch *patch)
+{
+	return git_patch_print(patch, diff_print_to_buffer_cb, out);
+}
+
+/* print a git_patch to a char* */
 int git_patch_to_str(
 	char **string,
 	git_patch *patch)
@@ -460,7 +468,7 @@ int git_patch_to_str(
 	int error;
 	git_buf output = GIT_BUF_INIT;
 
-	if (!(error = git_patch_print(patch, diff_print_to_buffer_cb, &output)))
+	if (!(error = git_patch_to_buf(&output, patch)))
 		*string = git_buf_detach(&output);
 	else {
 		git_buf_free(&output);