[bugfix][building]Failing installer build from git flow release or any branch containing file separator

Correction to solve failing installer build from any gitflow release branch. eg: release/eXist-2.2-Dev.RC1
 
Modification git.branch to replace any \ or  / by -.

Those \ and  / characters could be interpreted as file separator.  

Property git.commit, based on git.branch, is used as part of installer target filename. By consequence, the build path refer an unexpected and inexisting subdirectory.