Only read unbuffered when we read the whole file

[martin-fleck-at]

What it does

We only want to use unbuffered reading when we read the whole file, otherwise the optimization we gain through it seems to be lost.

Please note that there is a difference between readFile which returns a binary buffer and read which is used to read textual content. Furthermore, limits are applied automatically on the read method so even if you only want to read 10 characters it will fail if the file size is larger than the limit (files.maxFileSizeMB preference).

Fixes #13161

How to test

• Open any file with the FileService and provide a specific length to read, see bug report
• You can use https://github.com/eclipse-theia/theia/tree/issues/13161-test which simply reads a length of 10 with read and readFile using the SampleCommand2.
• Both should work without issues, even if you choose a large enough file as I set the limit option of the read method to the highest, safe integer.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[tortmayr]

Changes look good to me.The Test Scenario now works as expected.