commands/operator-sdk/cmd: quiet initial git commit

[estroz]

fatal: write failure on 'stdout': Bad file descriptor error is encountered by some users because the call to git commit uses a nil output fd. This commit allows git commit to print to stdout but quietly, fixing the error.

Fixes #363

[hasbro17]

LGTM.
Btw can you share the output of what gets printed out on operator-sdk new with this.
I'm assuming with -q nothing is printed.

[estroz]

Yes, nothing prints when the binary calls git commit.

[test@test operator-sdk-samples]$ operator-sdk new test-operator --api-version=test.example.com/v1 --kind=TestService
Create test-operator/.gitignore 
...
Run dep ensure done
Run code-generation for custom resources
Generating deepcopy funcs

Run git init ...
Initialized empty Git repository in /home/test/go/src/github.com/operator-framework/operator-sdk-samples/test-operator/.git/
Run git init done

[fanminshi]

lgtm

[rithujohn191]

👍 LGTM