Make all functions in buck-completion.bash usable

Created by: jbriales

Bash completion capabilities are nicely structured into different bash functions in https://github.com/facebook/buck/blob/master/scripts/buck-completion.bash though. This means completion functions like _buck_completion_try_build can be easily reused by users/developers for other custom functions beyond buck.

However the way logging currently works for bash completion, there is local log=_buck_completion_log locally defined in _buck_completion_run. This means if we call directly e.g. _buck_completion_try_build, log will be empty there and logging becomes execution (i.e. everything breaks).

This could be easily fixed by making log an environment variable, and since this is a very common name, we can rename log into sth like LOG_BUCK_COMPLETION. If that sounds right, I can do a pull request.