Do not select the default key with --ssh=agent

[olegmlsn]

should address #170

If no ssh key is specified and the --ssh-agent option is specified, do not select the default key

[umputun]

thx. a small thing what is missing - a small test case for the change

[umputun]

re: linter thing - I have updated master, pls rebase your branch and it should pass

[coveralls]

Pull Request Test Coverage Report for Build 7890943636

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 84.427%

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Totals
Change from base Build 7795981426:	0.01%
Covered Lines:	2651
Relevant Lines:	3140

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💛 - Coveralls

[umputun]

LGTM, thx