connect_work_tree_and_git_dir: safely create leading directories

connect_work_tree_and_git_dir: safely create leading directories

In a later patch we'll use connect_work_tree_and_git_dir when the
directory for the gitlink file doesn't exist yet. This patch makes
connect_work_tree_and_git_dir safe to use for both cases of
either the git dir or the working dir missing.

To do so, we need to call safe_create_leading_directories[_const]
on both directories. However this has to happen before we construct
the absolute paths as real_pathdup assumes the directories to
be there already.

So for both the config file in the git dir as well as the .git link
file we need to
a) construct the name
b) call SCLD
c) get the absolute path
d) once a-c is done for both we can consume the absolute path
   to compute the relative path to each other and store those
   relative paths.

The implementation provided here puts a) and b) for both cases first,
and then performs c and d after.

One of the two users of 'connect_work_tree_and_git_dir' already checked
for the directory being there, so we can loose that check as
connect_work_tree_and_git_dir handles this functionality now.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>