Use bookmark terminology if `arc land` on hg bookmark

Summary: Changes message for `arc land` that displays current branch or bookmark (if none is specified) to appropriately use the term 'bookmark' when on a bookmark in an hg repository.

Test Plan:
Ran `arc land` on new git and hg repositories, checking for correct identification of 'branch' or 'bookmark'.

~/test$ mkdir hg-test
~/test$ mkdir git-test
~/test$ cd hg-test
~/test/hg-test$ hg init
~/test/hg-test$ hg branch
default
~/test/hg-test$ hg bookmarks
no bookmarks set
~/test/hg-test$ arc land
Landing current branch 'default'.
Usage Exception: You can not land a branch onto itself -- you are trying to land 'default' onto 'default'. For more information on how to push changes, see 'Pushing and Closing Revisions' in 'Arcanist User Guide: arc diff' in the documentation. You may be able to 'arc amend' instead.
~/test/hg-test$ hg bookmark testmark
~/test/hg-test$ hg bookmarks
 * testmark                  -1:000000000000
~/test/hg-test$ hg branch
default
~/test/hg-test$ arc land
Landing current bookmark 'testmark'.
Usage Exception: Source testmark is a bookmark but destination default is not a bookmark. When landing a bookmark, the destination must also be a bookmark. Use --onto to specify a bookmark, or set arc.land.onto.default in .arcconfig.

Confirm still works on a git branch:

~/test/hg-test$ cd ../git-test/
~/test/git-test$ ls
~/test/git-test$ git init
Initialized empty Git repository in ~/test/git-test/.git/
~/test/git-test$ touch testfile
~/test/git-test$ git commit -am 'Test file'
~/test/git-test$ git branch
* master
~/test/git-test$ arc land
Landing current branch 'master'.
Usage Exception: You can not land a branch onto itself -- you are trying to land 'master' onto 'master'. For more information on how to push changes, see 'Pushing and Closing Revisions' in 'Arcanist User Guide: arc diff' in the documentation. You may be able to 'arc amend' instead.

Reviewers: DurhamGoode, epriestley

Reviewed By: DurhamGoode

CC: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D5163