Revert "match the whole command for git completion"

This reverts commit d957d23d8f.
Use __fish_git_using_command to test commands and __fish_contains_opt to
test arguments.

share/completions/git.fish

@@ -43,8 +43,7 @@ end
 function __fish_git_using_command
   set cmd (commandline -opc)
   if [ (count $cmd) -gt 1 ]
-    set -e cmd[1]
-    if [ (echo $argv) = (echo $cmd) ]
+    if [ $argv[1] = $cmd[2] ]
       return 0
     end
   end