t1300: check stderr for "ignores pairs" tests

t1300: check stderr for "ignores pairs" tests

Tests "git config ignores pairs ..." in t1300-config.sh validate that
"git config" ignores various kinds of supplied pairs of environment
variables GIT_CONFIG_KEY_* GIT_CONFIG_VALUE_* depending on
GIT_CONFIG_COUNT.  By "ignores" here we mean that "git config" abides by
the value of environment variable GIT_CONFIG_COUNT and doesn't use
key-value pairs outside of the supplied GIT_CONFIG_COUNT when trying to
produce a value for config key "pair.one".

These tests also validate that "git config" doesn't complain about
mismatched environment variables to standard error.  This is validated
by redirecting the standard error to a file called "error" and asserting
that it is empty.  However, two of these tests incorrectly redirect to
standard output while calling the file "error", and test 'git config
ignores pairs exceeding count' doesn't validate standard error at all.

Fix these tests by redirecting standard error to file "error" and
asserting its emptiness.

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>