Stacked Git 1.5

Several pesky bugs repaired along with some other minor improvements.

Thanks to everyone who submitted a PR or reported an issue!

Added

• Add Makefile targets for installing shell completions
• stg rebase --interactive learns 'hide' instruction

Changed

• Picked patch names are preserved when possible (#175)
• Replace --unapplied option with --noapply for stg pick (#174)
• stg pick --noapply no longer reverses patch order (#174)
• Use stg version uses sys.executable to get Python version.

Fixed

• Repair stg repair with amended first patch (#163)
• Repair corner cases where invalid patchnames could be generated by
  stg new, stg uncommit, etc. (#176)
• stg mail could crash due to a misspelled reference (#178)
• Zsh completion for stg refresh -p now completes against all patches
  (not just applied patches).
• Zsh gains missing completion for stg push --noapply
• Minor repair to help for stg float --noapply and stg push --noapply
• Restore stg sink --nopush capability.