Change --list to only search for secrets section and not everything

[mobilutz]

Description of changes:

I stumbled over a problematic situation, because my git config had some content with secrets in it. The config that had this in it is:

[branch "ll-INV-add-git-secrets"]
	remote = origin
	merge = refs/heads/ll-INV-add-git-secrets

When running git secrets --list, it showed me two files because the regex is not looking for the section but just for any secrets.* string.

With this change, git secrets --list only returns the configuration in the secrets section of git

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sparr]

Good catch on the misbehavior and fix! The pattern should also have a literal dot, so it won't match secretsfoo. However, this will be addressed more thoroughly by #215, so I'm going to close this PR. Thank you for the contribution, though.