[git wrapper] defend against helpful git

When using `GIT_SSH`, git attempts to detect accommodate the use of
`Plink.exe` by simply matching against `plink`. If detected, git adds
`-batch` to the command invocation which breaks our wrapper scripts.

Filter this specific case. Ref: Triage-5683, US729
Merge pull request #104 from engineyard/plink

[git wrapper] defend against helpful git
Support experimental ey.yml options `keep_releases` & `keep_failed_releases`

Changes the number of releases or failed releases that will be kept
after the deploy has finished.
Warn if we skip a non-executable "executable" hook.

We only execute hooks if they have both the correct name and are
executable. If there's a file that has a hook-like name but is not
executable we still don't run it, but now we output a message so that
it's more obvious why it has not been executed.
Merge pull request #107 from engineyard/warn-exec-deploy-hook-not-executable

Warn if we skip a non-executable "executable" hook.
Merge pull request #106 from engineyard/keep-releases

Support experimental ey.yml options `keep_releases` & `keep_failed_releases`
Show a warning (not status) when an apparent executable deploy hook is skipped
Vendor json_pure to keep multi_json from complaining about ok_json