devel/git-cinnabar: trim dead code inherited by Git subcommands

$ size /usr/local/libexec/git-core/git-cinnabar-helper

text    data      bss       dec        hex   filename

• 1858008 28376 217784 2104168 0x201b68 /usr/local/libexec/git-core/git-cinnabar-helper

+ 1154131 26704 207992 1388827 0x15311b /usr/local/libexec/git-core/git-cinnabar-helper