[PATCH 06/19] t6300: make `%(raw:size) --shell` test more robust

This test populates its `expect` file solely by appending content but fails to ensure that the file starts out empty. The test succeeds only because no earlier test populated a file of the exact same name, however this is an accident waiting to happen. Make the test more robust by ensuring that it contains exactly the intended content.

While at it, simplify the implementation via a straightforward `sed` application and by avoiding dropping out of the single-quote context within the test body (thus eliminating a hard-to-digest combination of apostrophes and backslashes).

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
---

Notes:
    An alternative simple fix would be to capture the output of the
    `while` loop itself rather than the output of `echo`:
    
        git for-each-ref ... | while read line
        do
            echo ...
        done >expect &&
    
    however, I prefer the conciseness of the `sed` approach,
    
    This patch makes no attempt to address `git` being upstream in a
    pipe. There are enough other such instances in this script that
    fixing them warrants a separate patch (if someone wants to tackle
    it).

 t/t6300-for-each-ref.sh | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)