alias: use run_command api to execute aliases

alias: use run_command api to execute aliases

On Windows, system() executes with cmd.exe instead of /bin/sh. This
means that aliases currently has to be batch-scripts instead of
bourne-scripts. On top of that, cmd.exe does not handle single quotes,
which is what the code-path currently uses to handle arguments with
spaces.

To solve both problems in one go, use run_command_v_opt() to execute
the alias. It already does the right thing prepend "sh -c " to the
alias.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>