git-gui: fix git-gui crash due to uninitialized variable

Recently, a clone initiated via git gui on Windows crashed on me due to
an "unknown variable cdone". It turns out that there is a code path
where this variable is used uninitialized.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

lib/status_bar.tcl

@@ -77,6 +77,7 @@ method start {msg uds} {
 
 method update {have total} {
 	set pdone 0
+	set cdone 0
 	if {$total > 0} {
 		set pdone [expr {100 * $have / $total}]
 		set cdone [expr {[winfo width $w_c] * $have / $total}]