Support for DesignSync (dssc) version control system

[Anu256]

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?

[jamessan]

I think it would be better to make the plugin extensible, as @mhinz has previously suggested.

Adding support for VCS we've never heard of and can't test doesn't sound like a good idea.