Place wrapper around comment as diff to catch panics (#15085) (#15094)

Backport #15085

There are a few recurrent issues with comment as diff reporting panics that are resistant to fixing due to the fact that the panic occurs in the template render and is swallowed by the template renderer.

This PR just adds some logging to force the panic to properly logged and re-propagates back up to the template renderer so we can actually detect what the issue is.

Signed-off-by: Andrew Thornton art27@cantab.net

services/gitdiff/gitdiff.go

@@ -1288,6 +1288,14 @@ func CommentAsDiff(c *models.Comment) (*Diff, error) {
 
 // CommentMustAsDiff executes AsDiff and logs the error instead of returning
 func CommentMustAsDiff(c *models.Comment) *Diff {
+	if c == nil {
+		return nil
+	}
+	defer func() {
+		if err := recover(); err != nil {
+			log.Error("PANIC whilst retrieving diff for comment[%d] Error: %v\nStack: %s", c.ID, err, log.Stack(2))
+		}
+	}()
 	diff, err := CommentAsDiff(c)
 	if err != nil {
 		log.Warn("CommentMustAsDiff: %v", err)