uscan: use git upstream remote when available in git mode

Yaddrequested to merge
yadd/devscripts:uscan-910777 into master

I found it stupid to clone upstream repository to a tmp repository when "git-buildpackage" is used with one of --git-upstream* options and mode=git.

This MR checks if upstream link exists using git remote --verbose show, then use it instead of cloning elsewhere; otherwise it falls back to previous behavior.

Closes: #910777

Edited by Yadd

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