Submodules: use relative paths as worktree and gitdir instead of absolute ones

[eder-s]

Description

Support relative paths for:

• a submodule's worktree configuration entry
• a submodule's gitdir link

Motivation

Using relative paths allows copying the super project to a different locations w/o worrying about the submodules. With absolute paths the submodules do still point to the "old" location after the superpoject is copied.

Alternatives considered

No response

Additional context

This issue was addressed previously at: https://bugs.eclipse.org/bugs/show_bug.cgi?id=463437

"cgit" also uses relative paths for worktree and gitdir.

[tomaswolf]

This is at

https://github.com/eclipse-jgit/jgit/blob/299a7348eb318a0199226c1e633cc46c659d76d3/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java#L296-L304

Reading core.worktree and the .git file already resolves them.

[msohn]

Simon proposed a fix in https://eclipse.gerrithub.io/c/eclipse-jgit/jgit/+/1198661