Do not assume default ref to be master

[trieloff]

helix-shared/src/GitUrl.js

Line 19 in 640e6ba

const DEFAULT_BRANCH = 'master';

the assumption is no longer valid and we may need a compatibility-breaking change here.

Related:
adobe/helix-publish#617
adobe/helix-pages#491
adobe/helix-resolve-git-ref#287

[tripodsan]

new location is

helix-shared/packages/helix-shared-git/src/GitUrl.js

Line 19 in 200d590

const DEFAULT_BRANCH = 'master';