The reason for the 'preview' is in the msysGit repo in the file HowToRelease.txt.
I think that msysGit is getting close to meeting the criteria suggested of:
A setup executable is named as follows
- it starts with Git;
- followed by the official git version if the release is closely
related to a specific version of official git;
- followed by
* preview<date>, for snapshot builds.
* winrc#, if we reach a certain quality level and plan to release
a stable version. If no bugs are found, this can be upgraded
to a stable release. We use winrc instead of simply rc to avoid
confusion with official release candidates. The prefix "win"
clearly indicates that the tag is about the Windows release.
Given the general quality and stability of msysGit it may be that there is an alternative that better reflects that feeling on the msysGit list msysgit@googlegroups.com