Use underscores instead of hyphens as bookmark suffixes.

Summary:
When trying to find an unused bookmark we append hyphened suffixes to the end of the bookmark name. In Mercurial, these are treated the same as the bookmark name without the suffix, here's the output from hg log -r:

```
[mehdi] HGPLAIN=1 hg log -r "arcpatch-1"
abort: unknown revision 'arcpatch'!
[mehdi] HGPLAIN=1 hg log -r "arcpatch_1"
abort: unknown revision 'arcpatch_1'!
```

Test Plan: None.

Reviewers: epriestley, dschleimer

Reviewed By: dschleimer

CC: aran, Korvin, DurhamGoode, dschleimer

Differential Revision: https://secure.phabricator.com/D6698
Merge branch 'master' of http://github.com/facebook/arcanist