"git rev-parse --show-superproject-working-tree" fails with "fatal: BUG: returned path string doesn't match cwd?" if supermodule has unmerged changes of the submodule reference

[Sam McKelvie]

I experienced this problem using git 2.17.1; however, from inspection of the 
next branch, function get_superproject_working_tree() in submodule.c has not 
changed in 2 years.

I believe the problem is related to the fact that when a merge of the submodule 
reference is in progress, "git --stage —full-name <submodule-relative-path>” 
returns three seperate entries for the submodule (one for each stage) rather 
than a single entry; e.g.,

$ git ls-files --stage --full-name submodule-child-test
160000 dbbd2766fa330fa741ea59bb38689fcc2d283ac5 1       submodule-child-test
160000 f174d1dbfe863a59692c3bdae730a36f2a788c51 2       submodule-child-test
160000 e6178f3a58b958543952e12824aa2106d560f21d 3       submodule-child-test

The code in get_superproject_working_tree() uses the “-z” option on ls-files, 
so it expects null-byte termination between entries. However, the computation 
of super_sub_len:

                super_sub_len = sb.buf + sb.len - super_sub - 1;

will only work when there is exactly one entry returned. If this line is 
changed to:

                super_sub_len = strlen(super_sub);

then only the first returned entry is used, and the bug is resolved.

strlen() should be safe to use here because strbuf_read ensures the result 
buffer is null-terminated.

Thanks,
Sam McKelvie