light bug in "git diff --shortstat" parsing

the string substitution assumed "insertions" was always plural, but git
actually takes care of suffixing insertion with an "s" only if there is
more than one insertion
fixes the parsing of git --shortstat

now the parsing work either there is only deletion or only insertions or both
Merge pull request #191 from jaesivsm/develop

light bug in "git diff --shortstat" parsing