Minor bug, git ls-files -o aborts because of broken submodules

$ git init abc
$ cd abc
$ mkdir def
$ echo 'gitdir: blah blah' >def/.git
$ git ls-files -o
fatal: Not a git repository: def/blah blah

If some directory looks like a submodule but turns out not, that's not
a fatal error. The stack trace is something like this. I suspect
do_submodule_path should use the gently version..

#1  0x0000000000588a78 in die
#2  0x0000000000558ded in read_gitfile_gently
#3  0x000000000051e2f6 in do_submodule_path
#4  0x000000000051e484 in git_pathdup_submodule
#5  0x00000000005340ac in resolve_gitlink_ref_recursive
#6  0x00000000005342cf in resolve_gitlink_ref
#7  0x00000000004dd20d in treat_directory
#8  0x00000000004dd760 in treat_one_path
#9  0x00000000004dd971 in treat_path
#10 0x00000000004de038 in read_directory_recursive
-- 
Duy