Skylark rule for new_git_repository doesn't work for repos with a 'build' directory on MacOS

[jmillikin-stripe]

MacOS has a case-insensitive filesystem, which means mkdir build; echo "" > BUILD fails. This prevents the Skylark rules in tools/build_defs/repo/git.bzl from working with external repositories that have an existing build/ directory for legacy reasons.

In #552, a workaround was added to Bazel such that a file BUILD.bazel is also acceptable. Can you please change git.bzl to use this filename when writing its stub build rules?

def _new_git_repository_implementation(ctx):
  # ...
  if ctx.attr.build_file:
    ctx.symlink(ctx.attr.build_file, 'BUILD.bazel')
  else:
    ctx.file('BUILD.bazel', ctx.attr.build_file_content)