blob: GetBlobContent: reduce allocations (#8223)

See #8222 for context.

## git.Blob.NewTruncatedReader

This introduce a new `NewTruncatedReader` method to return a blob-reader which silently truncates when the limit is reached (io.EOF will be returned).
Since the actual size is also returned `GetBlobContent` can pre-allocate a `[]byte` of the full-size (min of the asked size and the actual size) and call `io.ReadFull(rc, buf)` (instead of `util.ReadWithLimit(dataRc, int(limit))` which is convoluted and not used anywhere else).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.

### Documentation

- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8223
Reviewed-by: Lucas <sclu1034@noreply.codeberg.org>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

modules/util/io.go

@@ -4,7 +4,6 @@
 package util
 
 import (
-	"bytes"
 	"errors"
 	"io"
 )
@@ -20,42 +19,6 @@ func ReadAtMost(r io.Reader, buf []byte) (n int, err error) {
 	return n, err
 }
 
-// ReadWithLimit reads at most "limit" bytes from r into buf.
-// If EOF or ErrUnexpectedEOF occurs while reading, err will be nil.
-func ReadWithLimit(r io.Reader, n int) (buf []byte, err error) {
-	return readWithLimit(r, 1024, n)
-}
-
-func readWithLimit(r io.Reader, batch, limit int) ([]byte, error) {
-	if limit <= batch {
-		buf := make([]byte, limit)
-		n, err := ReadAtMost(r, buf)
-		if err != nil {
-			return nil, err
-		}
-		return buf[:n], nil
-	}
-	res := bytes.NewBuffer(make([]byte, 0, batch))
-	bufFix := make([]byte, batch)
-	eof := false
-	for res.Len() < limit && !eof {
-		bufTmp := bufFix
-		if res.Len()+batch > limit {
-			bufTmp = bufFix[:limit-res.Len()]
-		}
-		n, err := io.ReadFull(r, bufTmp)
-		if err == io.EOF || err == io.ErrUnexpectedEOF {
-			eof = true
-		} else if err != nil {
-			return nil, err
-		}
-		if _, err = res.Write(bufTmp[:n]); err != nil {
-			return nil, err
-		}
-	}
-	return res.Bytes(), nil
-}
-
 // ErrNotEmpty is an error reported when there is a non-empty reader
 var ErrNotEmpty = errors.New("not-empty")