Add Pull Request merge options - Ignore white-space for conflict checking, Rebase, Squash merge (#3188)

* Pull request options migration and UI in settings

* Add ignore whitespace functionality

* Fix settings if pull requests are disabled

* Fix migration transaction

* Merge with Rebase functionality

* UI changes and related functionality for pull request merging button

* Implement squash functionality

* Fix rebase merging

* Fix pull request merge tests

* Add squash and rebase tests

* Fix API method to reuse default message functions

* Some refactoring and small fixes

* Remove more hardcoded values from tests

* Remove unneeded check from API method

* Fix variable name and comment typo

* Fix reset commit count after PR merge

routers/repo/setting.go

@@ -211,7 +211,12 @@ func SettingsPost(ctx *context.Context, form auth.RepoSettingForm) {
 			units = append(units, models.RepoUnit{
 				RepoID: repo.ID,
 				Type:   models.UnitTypePullRequests,
-				Config: new(models.UnitConfig),
+				Config: &models.PullRequestsConfig{
+					IgnoreWhitespaceConflicts: form.PullsIgnoreWhitespace,
+					AllowMerge:                form.PullsAllowMerge,
+					AllowRebase:               form.PullsAllowRebase,
+					AllowSquash:               form.PullsAllowSquash,
+				},
 			})
 		}