Fix #393: add flag to dereference links

[agrzeslak]

Fixes: #393

There is currently no equivalent to ls's -L/--dereference flag. This is often undesirable, as it is rare that a user is interested in the details of the link itself. As such, the -X/--dereference flag has been added which implements this functionality. I have chosen -X as -L was already taken, and in my mind -X was similar to "across", in the sense that you are traversing "across" the link. The similar -H/--dereference-command-line flag from ls has also not been implemented in this PR, however it should be straightforward to add, and I may add it to this PR if I have some additional time or it is requested. I am more than happy to accommodate other suggestions, or change my implementation in any way that would be more acceptable to the maintainers.

Output without the -X flag (identical to before this PR):

cargo run -- -l
drwxr-xr-x   - root    23 Sep 14:39 dir
.rw-r--r-- 450 root    23 Sep 12:24 file.txt
lrwxrwxrwx  14 andrzej 27 Sep 14:17 symlink-loop-0 -> symlink-loop-1
lrwxrwxrwx  14 andrzej 27 Sep 14:17 symlink-loop-1 -> symlink-loop-0
lrwxrwxrwx  20 andrzej 23 Sep 12:26 symlink-to-another-file -> dir/another-file.txt
lrwxrwxrwx   3 andrzej 23 Sep 12:26 symlink-to-dir -> dir
lrwxrwxrwx   8 andrzej 23 Sep 12:23 symlink-to-file -> file.txt
lrwxrwxrwx  15 andrzej 27 Sep 14:15 symlink-to-symlink-to-file -> symlink-to-file

Output with the -X flag (links are now dereferenced):

cargo run -- -lX
drwxr-xr-x   - root 23 Sep 14:39 dir
.rw-r--r-- 450 root 23 Sep 12:24 file.txt
----------   - -    -            symlink-loop-0 -> symlink-loop-1
----------   - -    -            symlink-loop-1 -> symlink-loop-0
lrw-r--r--   0 root 27 Sep 16:16 symlink-to-another-file -> dir/another-file.txt
lrwxr-xr-x   - root 23 Sep 14:39 symlink-to-dir -> dir
lrw-r--r-- 450 root 23 Sep 12:24 symlink-to-file -> file.txt
lrw-r--r-- 450 root 23 Sep 12:24 symlink-to-symlink-to-file -> symlink-to-file

Output when using the analogous -L flag within ls:

ls -lL
ls: cannot access 'symlink-loop-0': Too many levels of symbolic links
ls: cannot access 'symlink-loop-1': Too many levels of symbolic links
total 20
drwxr-xr-x 2 root root 4096 Sep 23 14:39 dir
-rw-r--r-- 1 root root  450 Sep 23 12:24 file.txt
l????????? ? ?    ?       ?            ? symlink-loop-0
l????????? ? ?    ?       ?            ? symlink-loop-1
-rw-r--r-- 1 root root    0 Sep 27 16:16 symlink-to-another-file
drwxr-xr-x 2 root root 4096 Sep 23 14:39 symlink-to-dir
-rw-r--r-- 1 root root  450 Sep 23 12:24 symlink-to-file
-rw-r--r-- 1 root root  450 Sep 23 12:24 symlink-to-symlink-to-file

[bd-g]

Updates? This seems like a good feature to bring it more in parity with ls

[pierzchalski]

I too would find this useful. Anything blocking this (beyond the merge conflicts)?