t6050-replace: don't disable stdin for the whole test script

The test script 't6050-replace.sh' starts off with redirecting the
whole test script's stdin from /dev/null.  This redirection has been
there since the test script was introduced in a3e82672
(replace_object: add a test case, 2009-01-23), but the commit message
doesn't explain why it was deemed necessary.  AFAICT, it has never
been necessary, and t6050 runs just fine and succeeds even without it,
not only the current version but past versions as well.

Besides being unnecessary, this redirection is also harmful, as it
prevents the test helper functions 'test_pause' and 'debug' from
working properly in t6050, because we can't enter any commands to the
shell and the debugger, respectively.

So let's remove that redirection.
Signed-off-by: NSZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t6050-replace.sh

@@ -4,8 +4,6 @@
 #
 test_description='Tests replace refs functionality'
 
-exec </dev/null
-
 . ./test-lib.sh
 . "$TEST_DIRECTORY/lib-gpg.sh"