[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.