Support for DesignSync (dssc) version control system

Nobody has claimed this yet.

Hi mhinz,
Currently, this plugin doesn't have "dssc" (ENOVIA Synchronicity DesignSync) version control support.
To support this plugin for dssc, I have added the following piece of code locally in my system.
```
./autoload/sy/repo.vim:182:" s:check_diff_dssc {{{1
./autoload/sy/repo.vim:183:function! s:check_diff_dssc(exitval, diff) abort
./autoload/sy/repo.vim:626: \ 'dssc': 'dssc diff -unified -c %f',
./doc/signify.txt:157: dssc
./doc/signify.txt:197: \ 'dssc': 'dssc diff -unified -c %f',
./doc/signify.txt:325:are: svn,dssc,bzr, darcs, fossil.
```
previously, I had the below changes.

```
./autoload/sy/repo.vim:148: " Function: #get_diff_dssc {{{1
./autoload/sy/repo.vim:149: function! sy#repo#get_diff_dssc(sy, exitval, diff) abort
./autoload/sy/repo.vim:150: call sy#verbose('get_diff_dssc()', 'dssc')
./autoload/sy/repo.vim:151: let [found_diff, diff] = a:exitval ? [0, []] : [1, s:strip_context(a:diff)]
./autoload/sy/repo.vim:152: call s:get_diff_end(a:sy, found_diff, 'dssc', diff)
./autoload/sy/repo.vim:153: endfunction
./autoload/sy/repo.vim:399: \ 'dssc': 'dssc',
./autoload/sy/repo.vim:410: echomsg 'signify: No diff tool found -> no support for svn, dssc, darcs, bzr, fossil.'
./autoload/sy/repo.vim:431: \ 'dssc': 'dssc diff -unified -c %f',
```
I realized the plugin may go through underlying changes and it would be good to have it part of the fork.
I don't have experience using git and plugin's internal details much. So I am requesting you to add this feature.
Can you set up the dssc feature so that any future changes will be taken care of?

Start with autoload/sy/repo.vim, reviewing the existing repository detection and diff-handling paths alongside the proposed dssc changes. Check the matching dssc configuration examples in doc/signify.txt and compare them with other supported VCSs. Done means dssc detection, diff handling, and documented configuration work consistently with future plugin changes.

Written by the indexing model from the issue text.