t5404: relax overzealous test

In 0b294c0abf (make deleting a missing ref more quiet, 2008-07-08), we
added a test to verify that deleting an already-deleted ref does not
show an error.

Our test simply looks for the substring 'error' in the output of the
`git push`, which might look innocuous on the face of it.

Suppose, however, that you are a big fan of whales. Or even better: your
IT administrator has a whale of a time picking cute user names, e.g.
referring to you (due to your like of India Pale Ales) as "one of the
cuter rorquals" (see https://en.wikipedia.org/wiki/Rorqual to learn a
thing or two about rorquals) and hence your home directory becomes
/home/cuterrorqual. If you now run t5404, it fails! Why? Because the
test calls `git push origin :b3` which outputs:

    To /home/cuterrorqual/git/t/trash directory.t5404-tracking-branches/.
     - [deleted]         b3

Note how there is no error displayed in that output? But of course
"error" is a substring of "cuterrorqual". And so that `grep error
output` finds something.

This bug was not, actually, caught having "error" as a substring of the
user name but while working in a worktree called "colorize-push-errors",
whose name was part of that output, too, suggesting that not even
testing for the *word* `error` via `git grep -w error output` would fix
the underlying issue.

This patch chooses instead to look for the prefix "error:" at the
beginning of the line, so that there can be no ambiguity that any catch
was indeed a message generated by Git's `error_builtin()` function.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5404-tracking-branches.sh

@@ -56,7 +56,7 @@ test_expect_success 'deleted branches have their tracking branches removed' '
 test_expect_success 'already deleted tracking branches ignored' '
 	git branch -d -r origin/b3 &&
 	git push origin :b3 >output 2>&1 &&
-	! grep error output
+	! grep "^error: " output
 '
 
 test_done