t3200: don't grep for `strerror()` string

t3200: don't grep for `strerror()` string

In 6b7093064a ("t3200: test for specific errors", 2020-06-15), we
learned to grep stderr to ensure that the failing `git branch`
invocations fail for the right reason. In two of these tests, we grep
for "File exists", expecting the string to show up there since config.c
calls `error_errno()`, which ends up including `strerror(errno)` in the
error message.

But as we saw in 4605a73073 ("t1091: don't grep for `strerror()`
string", 2020-03-08), there exists at least one implementation where
`strerror()` yields a slightly different string than the one we're
grepping for. In particular, these tests fail on the NonStop platform.

Similar to 4605a73073, grep for the beginning of the string instead to
avoid relying on `strerror()` behavior.

Reported-by: Randall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>