Option to set default branch at repository creation (#10803)

* Option to set default branch at repository creation
* Handle template repos with non-default master branch
* Add DefaultBranch handling on creation to API

Fix #9542

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/repo.go

@@ -929,6 +929,7 @@ type CreateRepoOptions struct {
 	IssueLabels    string
 	License        string
 	Readme         string
+	DefaultBranch  string
 	IsPrivate      bool
 	IsMirror       bool
 	AutoInit       bool