Rename "stableCommitName" to "stableCommit"

Summary:
Ref T2683. This is closely related to "symbolicCommit", but has an inconsistent "name" on the end.

Also, diffusion.searchquery uses this parameter inconsistently.

Test Plan:

• greped for callsites.
• Ran searches in Git and Mercurial repositories.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9091