Merge branch 'sb/submodule-url-to-absolute'

Some codepaths failed to form a proper URL when .gitmodules record
the URL to a submodule repository as relative to the repository of
superproject, which has been corrected.

* sb/submodule-url-to-absolute:
submodule helper: convert relative URL to absolute URL if needed

diff --cc builtin/submodule--helper.c
Simple merge