Limit number of blobs fetched in a single call

What does this MR do?

When a large number of blobs requested, we'd want to fetch them in multiple Gitaly calls. The number 250 is calculated empirically:

• the number is small enough to fetch the data before the timeout
• the number is as big as possible in order to avoid making too many Gitaly calls

For example, we have a limit for a number of diff files fetched: 1000. In this case, we'll perform 4 Gitaly calls for blobs

Related issue: #31118 (closed)