Avoid running detection more than once

Before this change, it was possible that a buffer was disabled, when Sy was run
more than once at startup, e.g. because of `BufEnter` and `FocusGained`.

The first call would start the detection and the second call would disable the
buffer, because the first call hadn't changed the VCS type from "unknown" yet.

autoload/sy.vim

@@ -35,13 +35,14 @@ function! sy#start() abort
   if !exists('b:sy') || b:sy.path != sy_path
     call sy#verbose('Register new file.')
     let b:sy = {
-          \ 'path'  : sy_path,
-          \ 'buffer': bufnr(''),
-          \ 'active': 0,
-          \ 'vcs'   : 'unknown',
-          \ 'hunks' : [],
-          \ 'signid': 0x100,
-          \ 'stats' : [-1, -1, -1] }
+          \ 'path'  :    sy_path,
+          \ 'buffer':    bufnr(''),
+          \ 'active':    0,
+          \ 'detecting': 0,
+          \ 'vcs'   :    'unknown',
+          \ 'hunks' :    [],
+          \ 'signid':    0x100,
+          \ 'stats' :    [-1, -1, -1] }
     if get(g:, 'signify_disable_by_default')
       call sy#verbose('Disabled by default.')
       return
@@ -60,8 +61,12 @@ function! sy#start() abort
       call sy#verbose('Redetecting VCS.')
       call sy#repo#detect(1)
     else
-      call sy#verbose('No VCS found. Disabling.')
-      call sy#disable()
+      if get(b:sy, 'detecting')
+        call sy#verbose('Detection is already in progress.')
+      else
+        call sy#verbose('No VCS found. Disabling.')
+        call sy#disable()
+      endif
     endif
   else
     let job_id = get(b:, 'sy_job_id_'.b:sy.vcs)

autoload/sy/repo.vim

@@ -16,6 +16,7 @@ function! sy#repo#detect(do_register) abort
   endif
 
   for vcs in vcs_list
+    let b:sy.detecting += 1
     call sy#repo#get_diff_start(vcs, a:do_register)
   endfor
 endfunction
@@ -61,6 +62,8 @@ function! s:job_exit(bufnr, vcs, exitval, diff, do_register) abort
   if empty(sy)
     call sy#verbose(printf('No b:sy found for %s', bufname(a:bufnr)), a:vcs)
     return
+  elseif sy.vcs == 'unknown' && sy.active
+    let sy.detecting -= 1
   endif
   call sy#repo#get_diff_{a:vcs}(sy, a:exitval, a:diff, a:do_register)
   call setbufvar(a:bufnr, 'sy_job_id_'.a:vcs, 0)