Makefile: better support for multiple checkouts
Makefile: better support for multiple checkouts

Use `$(git rev-parse --git-path)` instead of hardcoding `.git`.

cockroachdb/cockroach,maxlang/cockroach,paperstreet/cockroach,veteranlu/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,petermattis/cockroach,glycerine/cockroach,paperstreet/cockroach,glycerine/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,maxlang/cockroach,maxlang/cockroach,petermattis/cockroach,veteranlu/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,cockroachdb/cockroach,maxlang/cockroach,glycerine/cockroach,glycerine/cockroach,cockroachdb/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,maxlang/cockroach,paperstreet/cockroach,paperstreet/cockroach,petermattis/cockroach,paperstreet/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,maxlang/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,paperstreet/cockroach,cockroachdb/cockroach,glycerine/cockroach,petermattis/cockroach,glycerine/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,petermattis/cockroach,veteranlu/cockroach,petermattis/cockroach,cockroachdb/cockroach