Support Git's safe.bareRepository=explicit config option #4141

Some environments configure GIt with the safe.bareRepository=explicit option in git-config for improved security.

With this setting Flutter's use of pub was failing due to this dependency on a package fetched through Git:
https://github.com/flutter/flutter/blob/master/dev/integration_tests/android_views/pubspec.yaml#L19

The pubspec.yaml specifies a Git URL and commit hash. The .pub-cache was storing this package as a Git bare repository.

With safe.bareRepository=explicit Pub fails to locate the commit hash:

Package not available (Could not find git ref '64d0f6051b9b7b9933d3d16194170a38f544634a'
(fatal: cannot use bare repository '/usr/local/google/home/jsimmons/.pub-cache/git/cache/goldens-9579e0161a174b307c855fff52478bf0dc30bb1a'
(safe.bareRepository is 'explicit'))).

Pub should support Git in this configuration. It may need to pass the --git-dir flag to Git when accessing these repositories in the cache.