Update 'arc land': add --squash, add default config, fix windows compat
Update 'arc land': add --squash, add default config, fix windows compat

Summary:
  - We have "--merge", added similar "--squash" for completeness.
  - Switched away from (cd .. && ...) pattern which is not functional on Windows.
  - Allow a default --onto to be specified, if master isn't the best default for a project.

Test Plan:
  - Did --hold --keep dry runs, changes landed properly.
  - Set a default onto branch in .arcconfig.
  - Ran --merge and --squash together to verify conflict configuration.

Reviewers: btrahan

CC: aran, epriestley

Maniphest Tasks: T124, T1033

Differential Revision: https://secure.phabricator.com/D2001

dguo-coursera/arcanist,Khan/arcanist,alexmv/arcanist,haskell-infra/arcanist,wxstars/arcanist,Symplicity/arcanist,wikimedia/phabricator-arcanist,codelabs-ch/arcanist,coursera/arcanist,eSpark/arcanist,dguo-coursera/arcanist,wxstars/arcanist,alexmv/arcanist,Symplicity/arcanist,KieranHayes/arcanist,dguo-coursera/arcanist,jra3/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,phacility/arcanist,aik099/arcanist,eSpark/arcanist,codelabs-ch/arcanist,KieranHayes/arcanist,PageUpPeopleOrg/arcanist-patches,aik099/arcanist,codelabs-ch/arcanist,alexmv/arcanist,hach-que/arcanist,coursera/arcanist,r4nt/arcanist,fivestars/arcanist,Khan/arcanist,Khan/arcanist,coursera/arcanist,zhihu/arcanist,phacility/arcanist,memsql/arcanist,hach-que/arcanist,KieranHayes/arcanist,uber/arcanist,memsql/arcanist,wharris/arcanist,sneakpeeq/arcanist,dguo-coursera/arcanist,folsom-labs/arcanist,wikimedia/phabricator-arcanist,memsql/arcanist,wharris/arcanist,phacility/arcanist,folsom-labs/arcanist,wxstars/arcanist,uber/arcanist,aik099/arcanist,hach-que/arcanist,Khan/arcanist,alexmv/arcanist,aik099/arcanist,kevinseelbach/arcanist,codelabs-ch/arcanist,r4nt/arcanist,KieranHayes/arcanist,Symplicity/arcanist,calblueprint/arcanist,jra3/arcanist,jra3/arcanist,codelabs-ch/arcanist,fivestars/arcanist,wharris/arcanist,PageUpPeopleOrg/arcanist-patches,uber/arcanist,julkiewicz/arcanist,kevinseelbach/arcanist,folsom-labs/arcanist,memsql/arcanist,PageUpPeopleOrg/arcanist-patches,sneakpeeq/arcanist,haskell-infra/arcanist,PageUpPeopleOrg/arcanist-patches,calblueprint/arcanist,calblueprint/arcanist,uber/arcanist,chundongwang/arcanist,calblueprint/arcanist,julkiewicz/arcanist,kevinseelbach/arcanist,eSpark/arcanist,hach-que/arcanist,kevinseelbach/arcanist,Symplicity/arcanist,fivestars/arcanist,haskell-infra/arcanist,haskell-infra/arcanist,sneakpeeq/arcanist,chundongwang/arcanist,wikimedia/phabricator-arcanist,wxstars/arcanist,folsom-labs/arcanist,julkiewicz/arcanist,zhihu/arcanist,fivestars/arcanist,jra3/arcanist,sneakpeeq/arcanist,zhihu/arcanist,coursera/arcanist,eSpark/arcanist,wharris/arcanist,chundongwang/arcanist,phacility/arcanist,chundongwang/arcanist,julkiewicz/arcanist,zhihu/arcanist,phacility/arcanist,r4nt/arcanist