Reduce noise in check-source-control-tools

Summary:
Currently, autopatch.sh may output errors which are expected while running
some of these tests. However, only the error code is checked and the output is not
necessary to capture. We can just ignore it and remove some noisy "Error: ..." style
output from showing even when the tests pass.

Test Plan:

cmake -GNinja -DBUILD_SOURCE_CONTROL_TOOLS=ON ..
ninja check-source-control-tools

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7355