Do not show full lfs file on error in git_test.go:rawTest() (#14980)

Do not show full lfs file on error in git_test.go:rawTest() (#14980)

If there is a problem uploading to LFS it is possible for the raw
endpoint to return a very large file when a pointer file is expected
This will then cause the drone logs to fill up unnecessarily with
the contents of the very large file.

If the file returned from raw is of the incorrect size we should
therefore not test it see if it contains the pointer file
and just declare that it is incorrect.

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