gitweb: Make it work with $GIT containing spaces

This fixes single point where $GIT (which can contain full path
to git binary) with embedded spaces gave errors.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -377,7 +377,7 @@ if (-e $GITWEB_CONFIG) {
 }
 
 # version of the core git binary
-our $git_version = qx($GIT --version) =~ m/git version (.*)$/ ? $1 : "unknown";
+our $git_version = qx("$GIT" --version) =~ m/git version (.*)$/ ? $1 : "unknown";
 
 $projects_list ||= $projectroot;