Use git_diff_get_stats in example/diff + refactor

This takes the `--stat` and related example options in the example
diff.c program and converts them to use the `git_diff_get_stats`
API which nicely formats stats for you.

I went to add bar-graph scaling to the stats formatter and noticed
that the `git_diff_stats` structure was holding on to all of the
`git_patch` objects.  Unfortunately, each of these objects keeps
the full text of the diff in memory, so this is very expensive.  I
ended up modifying `git_diff_stats` to keep just the data that it
needs to keep and allowed it to release the patches.  Then, I added
width scaling to the output on top of that.

In making the diff example program match 'git diff' output, I ended
up removing an newline from the sumamry output which I then had to
compensate for in the email formatting to match the expectations.

Lastly, I went through and refactored the tests to use a couple of
helper functions and reduce the overall amount of code there.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/diff.c b/src/diff.c
index 0d1aed4ad..fd881c6f6 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -1590,7 +1590,8 @@ int git_diff_format_email(
 
 	if ((error = git_buf_puts(out, "---\n")) < 0 ||
 		(error = git_diff_get_stats(&stats, diff)) < 0 ||
-		(error = git_diff_stats_to_buf(out, stats, format_flags)) < 0 ||
+		(error = git_diff_stats_to_buf(out, stats, format_flags, 0)) < 0 ||
+		(error = git_buf_putc(out, '\n')) < 0 ||
 		(error = git_diff_format_email__append_patches_tobuf(out, diff)) < 0)
 			goto on_error;