submodule foreach: skip eval for more than one argument

submodule foreach: skip eval for more than one argument

'eval "$@"' creates an extra layer of shell interpretation, which is
probably not expected by a user who passes multiple arguments to git
submodule foreach:

 $ git grep "'"
 [searches for single quotes]
 $ git submodule foreach git grep "'"
 Entering '[submodule]'
 /usr/lib/git-core/git-submodule: 1: eval: Syntax error: Unterminated quoted string
 Stopping at '[submodule]'; script returned non-zero status.

To fix this, if the user passes more than one argument, execute "$@"
directly instead of passing it to eval.

Examples:

 * Typical usage when adding an extra level of quoting is to pass a
   single argument representing the entire command to be passed to the
   shell.  This doesn't change that.

 * One can imagine someone feeding untrusted input as an argument:

 	git submodule foreach git grep "$variable"

   That currently results in a nonobvious shell code injection
   vulnerability.  Executing the command named by the arguments
   directly, as in this patch, fixes it.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>