plugin/git.vim: Save current buffer before commit.

Previously, GitCommit didn't save the current buffer, meaning that changes
since the last save weren't reflected in the commit. This fix is hopefully
more consistent with user expectations.

plugin/git.vim

@@ -125,13 +125,16 @@ endfunction
 " Commit.
 function! GitCommit(args)
     let git_dir = <SID>GetGitDir()
-
+    
     let args = a:args
 
     if args !~ '\v\k@<!(-a|--all)>' && s:SystemGit('diff --cached --stat') =~ '^\(\s\|\n\)*$'
         let args .= ' -a'
     endif
 
+    " Save current buffer so Git sees your changes
+    execute (":w")
+
     " Create COMMIT_EDITMSG file
     let editor_save = $EDITOR
     let $EDITOR = ''