Clear the SHA1 entry field when we go to paste something into it

If the user pastes in the selection (with the middle mouse button) and
it already has 40 characters in it, clear it before pasting.

gitk

@@ -398,6 +398,7 @@ proc makewindow {} {
     bind . <Button-1> "click %W"
     bind $fstring <Key-Return> dofind
     bind $sha1entry <Key-Return> gotocommit
+    bind $sha1entry <<PasteSelection>> clearsha1
 
     set maincursor [. cget -cursor]
     set textcursor [$ctext cget -cursor]
@@ -1566,6 +1567,13 @@ proc incrfont {inc} {
     redisplay
 }
 
+proc clearsha1 {} {
+    global sha1entry sha1string
+    if {[string length $sha1string] == 40} {
+	$sha1entry delete 0 end
+    }
+}
+
 proc sha1change {n1 n2 op} {
     global sha1string currentid sha1but
     if {$sha1string == {}