git submodule add requires remote

Dear gitsters, dear Lars,

while testing some workflows with submodules, we ran into the problem that adding a submodule seems to require a remote "origin" to be defined, even though that is not really necessary:

  ### subproj setup:

  % GIT_DIR=subproj.git git --bare init
  Initialized empty Git repository in /tmp/madduck/cdt.ciwkf2/subproj.git/
  % mkdir subproj && cd subproj
  % git init && git remote add origin ../subproj.git
  Initialized empty Git repository in /tmp/madduck/cdt.ciwkf2/subproj/.git/
  % touch a && git add a && git commit -minitial && git push origin master
  [master (root-commit) 7f6f6e9] initial
  0 files changed, 0 insertions(+), 0 deletions(-)
  create mode 100644 a
  Counting objects: 3, done.
  Writing objects: 100% (3/3), 201 bytes, done.
  Total 3 (delta 0), reused 0 (delta 0)
  Unpacking objects: 100% (3/3), done.
  To ../subproj.git
  * [new branch]      master -> master

  ### test project setup:

  % mkdir ../proj && cd ../proj
  % git init
  Initialized empty Git repository in /tmp/madduck/cdt.ciwkf2/proj/.git/
  % touch a && git add a && git commit -minitial
  [master (root-commit) aa15bae] initial
  0 files changed, 0 insertions(+), 0 deletions(-)
  create mode 100644 a

  ### adding submodule:

  % git submodule add ../subproj.git
  remote (origin) does not have a url defined in .git/config

The requirements stems from the resolve_relative_url() function in git-submodule, which assumes that a relative submodule URL is relative to the superproject's default remote URL. This might make sense for './' URLs, but not really for '../'. Plus, it breaks the normal assumption that Git accepts relative (filesystem) paths as URLs (works for git-clone etc.).

Before I work out a patch for this, I would be curious to hear your thoughts.

Thanks,