Issue 8211: Format block comments with tabs correctly (#8248)

Summary:
This PR fixes issue #8211. We tested it on a few different files, with only tabs, only spaces, and mixed tabs and spaces.

The asterisks aren't aligned in the block comment, but this is also currently true for files indented with only spaces.
Closes #8248

Differential Revision: D8707054

Pulled By: fredemmott

fbshipit-source-id: 559261d919bf2c192bd24eebf8fe1952915a63ae

hphp/hack/src/hackfmt/hack_format.ml

@@ -3060,7 +3060,7 @@ and transform_trivia ~is_leading trivia =
         let prefix_space_count str =
           let len = String.length str in
           let rec aux i =
-            if i = len || str.[i] <> ' '
+            if i = len || (str.[i] <> ' ' && str.[i] <> '\t')
             then 0
             else 1 + (aux (i + 1))
           in