* Use exec.CommandContext to simplfy timeout handling
And fixing the data races which can be identified by the added tests when -race enabled.
* Use sleep commmand instead of reading from stdin
* Make the error handling go-esque
* Add a process.Manager singleton with process.GetManager()
* Use process.GetManager everywhere
* Fix godoc comments for process module
* Increment process counter id after locking the mutex
Add a sibling to ExecDir which is capable of specifying environment variables,
so that we can invoke `git` with GIT_INDEX_FILE, GIT_DIR, etc..
For #258
