Refactors checking out and head sha parsing from the promoter to the
release change promoter

Signed-off-by: Mike Stallard <mstallard@pivotallabs.com>
refactors branch merging out of the promoter into a generic git branch merging object

Signed-off-by: Mike Stallard <mstallard@pivotallabs.com>
Refactors stable tag checking out of the promoter into the git tagger

Signed-off-by: Mike Stallard <mstallard@pivotallabs.com>
Bump curl version

Signed-off-by: Mike Stallard <mstallard@pivotallabs.com>