Check if ]C and [C are already mapped

Fixes #264

doc/signify.txt

@@ -512,7 +512,13 @@ Hunk jumping:~
     ]C   Jump to last hunk.
     [C   Jump to first hunk.
 
-If you don't like these mappings, you can map them yourself:
+These keys only get mapped by default when:
+
+  - The keys are not mapped already (by you or another plugin).
+  - There are no other keys that are mapped to do the same (to avoid duplicate
+    mappings).
+
+Mapping other keys:
 >
     nmap <leader>gj <plug>(signify-next-hunk)
     nmap <leader>gk <plug>(signify-prev-hunk)

plugin/signify.vim

@@ -85,13 +85,17 @@ nnoremap <silent> <expr> <plug>(signify-prev-hunk) &diff
       \ ? '[c'
       \ : ":\<c-u>call sy#jump#prev_hunk(v:count1)\<cr>"
 
-if empty(maparg(']c', 'n'))
+if empty(maparg(']c', 'n')) && !hasmapto('<plug>(signify-next-hunk)', 'n')
   nmap ]c <plug>(signify-next-hunk)
-  nmap ]C 9999]c
+  if empty(maparg(']C', 'n')) && !hasmapto('9999]c', 'n')
+    nmap ]C 9999]c
+  endif
 endif
-if empty(maparg('[c', 'n'))
+if empty(maparg('[c', 'n')) && !hasmapto('<plug>(signify-prev-hunk)', 'n')
   nmap [c <plug>(signify-prev-hunk)
-  nmap [C 9999[c
+  if empty(maparg('[C', 'n')) && !hasmapto('9999[c', 'n')
+    nmap [C 9999[c
+  end
 endif
 
 " hunk text object