Updating spec files for commit-based releases

[dvershinin]

When invoked against spec file, check existence of "commit" global. If such exists then fetch latest commit and update it in the spec file. Also update or add commit_date global. Update release field to:

• if no release or tag exists at all, set release to 0.snapinfo
• if there is any release, we are assuming that the latest commit represents a post-release, so 1.snapinfo

Snapinfo = git<commit date><short commit>

Example: https://github.com/gnachman/iTerm2-shell-integration