git-clone: improve error message if curl program is missing or not executable

git-clone: improve error message if curl program is missing or not executable

If the curl program is not available (or not executable), and git clone is
started to clone a repository through http, this is the output

 Initialized empty Git repository in /tmp/puppet/.git/
 /usr/bin/git-clone: line 37: curl: command not found
 Cannot get remote repository information.
 Perhaps git-update-server-info needs to be run there?

This patch improves the error message by checking the return code when
running curl to exit immediately if it's 126 or 127; the error output now
is

 Initialized empty Git repository in /tmp/puppet/.git/
 /usr/bin/git-clone: line 37: curl: command not found

Adrian Bridgett noticed this and reported through
 http://bugs.debian.org/440976

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>