submodule: die on config error when linking modules

submodule: die on config error when linking modules

When trying to connect a submodule with its corresponding
repository in '.git/modules' we try to set the core.worktree
setting in the submodule, which may fail due to an error
encountered in `git_config_set_in_file`.

The function is used in the git-mv command when trying to move a
submodule to another location. We already die when renaming a
file fails but do not pay attention to the case where updating
the connection between submodule and its repository fails. As
this leaves the repository in an inconsistent state, as well,
abort the program by dying early and presenting the failure to
the user.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>