Git.pm: Handle failed commands' output

Git.pm: Handle failed commands' output

Currently if an external command returns error exit code, a generic exception
is thrown and there is no chance for the caller to retrieve the command's
output.

This patch introduces a Git::Error::Command exception class which is thrown
in this case and contains both the error code and the captured command output.
You can use the new git_cmd_try statement to fatally catch the exception
while producing a user-friendly message.

It also adds command_close_pipe() for easier checking of exit status of
a command we have just a pipe handle of. It has partial forward dependency
on the next patch, but basically only in the area of documentation.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>