Fix for `@cron` triggers

[mach6]

Git repo paths do not start with "/"

Prior to this fix, it was possible that a @cron associated with the non-default branch would return the incorrect config data.

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This PR fixes the following error;

given a repo, with a default (typically master) branch, that contains some tree;

$ tree -a .                                                                        
.
└── README.md

0 directories, 1 file

and a @cron established for the same repo, configured for branch foo, which contains a different tree;

$ tree -a .                                                                   
.
├── component_a
│   └── .drone.yml
├── component_b
│   └── .drone.yml
└── README.md

2 directories, 3 files

if the latest commit ref for foo is abcde12345, and assuming a GitHub repository, the existing code would call repos/{slug}/contents//?ref=abcde12345 which would return the tree for the default branch. This is due to the extra / before the query string in the request. See the default when ref is omitted per the documentation - https://developer.github.com/v3/repos/contents/#get-contents

The expected behavior for a @cron (and with drone-tree-config) is to walk the entire tree for the branch configured in Drone.

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• In addition this PR applies the same fix when the change set does not include any changed files.
• Last, it removes the / prefix which was applied in the code for plugin.getConfigForChanges(...)

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Note: I have not tested this change with the bitbucket and gitlab SCM's. This said, the following documentation (and code) suggests that / as a prefix is also not expected, for them;

• The API called in the bitbucket integration -- https://developer.atlassian.com/bitbucket/api/2/reference/resource/repositories/%7Bworkspace%7D/%7Brepo_slug%7D/src/%7Bnode%7D/%7Bpath%7D
• The API called in the gitlab integration -- https://docs.gitlab.com/ce/api/repositories.html#list-repository-tree
• Current code for gitlab integration which removes the / prefix -- https://github.com/bitsbeats/drone-tree-config/blob/master/plugin/scm_clients/gitlab_client.go#L127

[foosinn]

Good catch, thanks a lot!

I'll release a new version once your other patch is fixed.