Show Branch Default

1st chunk of `Documentation/config/showbranch.adoc`

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showBranch.default::
	The default set of branches for linkgit:git-show-branch[1].
	See linkgit:git-show-branch[1].

Title: Show Branch Default

Summary

The default set of branches used by the git-show-branch command, with more information available in the git-show-branch documentation.