Allow for using the native engine from-source in another repo (#4105)
Allow for using the native engine from-source in another repo (#4105)

### Problem

Currently, running the pantsbuild/pants repo from source in another repo (ie, `cd ${other_repo}; ../pantsbuild/pants list ::`) is broken due to use of relative paths in file fingerprinting. We use this workflow internally to iterate on new unreleased pants code.

### Solution

Execute file fingerprinting in a subshell that cds to the buildroot of pantsbuild/pants. Note that using the `git --work-tree` option didn't work... possible due to reads of `.gitignore` files ignoring that option.
lahosken/pants,UnrememberMe/pants,cevaris/pants,benjyw/pants,twitter/pants,pombredanne/pants,cevaris/pants,tdyas/pants,pantsbuild/pants,twitter/pants,tdyas/pants,UnrememberMe/pants,cevaris/pants,pantsbuild/pants,UnrememberMe/pants,15Dkatz/pants,peiyuwang/pants,15Dkatz/pants,wisechengyi/pants,twitter/pants,jsirois/pants,15Dkatz/pants,landism/pants,UnrememberMe/pants,lahosken/pants,benjyw/pants,peiyuwang/pants,peiyuwang/pants,UnrememberMe/pants,baroquebobcat/pants,pantsbuild/pants,foursquare/pants,baroquebobcat/pants,baroquebobcat/pants,baroquebobcat/pants,fkorotkov/pants,peiyuwang/pants,landism/pants,landism/pants,pombredanne/pants,benjyw/pants,cevaris/pants,15Dkatz/pants,landism/pants,lahosken/pants,baroquebobcat/pants,pantsbuild/pants,wisechengyi/pants,tdyas/pants,fkorotkov/pants,landism/pants,15Dkatz/pants,foursquare/pants,foursquare/pants,benjyw/pants,foursquare/pants,tdyas/pants,twitter/pants,lahosken/pants,wisechengyi/pants,jsirois/pants,benjyw/pants,peiyuwang/pants,15Dkatz/pants,cevaris/pants,benjyw/pants,baroquebobcat/pants,UnrememberMe/pants,tdyas/pants,tdyas/pants,foursquare/pants,pombredanne/pants,15Dkatz/pants,mateor/pants,baroquebobcat/pants,baroquebobcat/pants,mateor/pants,fkorotkov/pants,fkorotkov/pants,pantsbuild/pants,lahosken/pants,peiyuwang/pants,jsirois/pants,mateor/pants,wisechengyi/pants,baroquebobcat/pants,lahosken/pants,wisechengyi/pants,fkorotkov/pants,wisechengyi/pants,wisechengyi/pants,mateor/pants,mateor/pants,mateor/pants,fkorotkov/pants,tdyas/pants,twitter/pants,UnrememberMe/pants,UnrememberMe/pants,pantsbuild/pants,lahosken/pants,wisechengyi/pants,lahosken/pants,tdyas/pants,pombredanne/pants,benjyw/pants,pombredanne/pants,landism/pants,peiyuwang/pants,twitter/pants,mateor/pants,foursquare/pants,landism/pants,fkorotkov/pants,pombredanne/pants,15Dkatz/pants,fkorotkov/pants,cevaris/pants,mateor/pants,pombredanne/pants,landism/pants,twitter/pants,wisechengyi/pants,pantsbuild/pants,foursquare/pants,twitter/pants,foursquare/pants,peiyuwang/pants,cevaris/pants,tdyas/pants,twitter/pants,foursquare/pants,UnrememberMe/pants