git-gui: Label the uncommitted blame history entry

If the user runs the blame viewer on a working directory file
instead of a specific commit-ish then we have no value for the
commit SHA1 or the summary line; this causes the history menu
to get an empty entry at the very bottom.  We now look for this
odd case and call the meny entry "Working Directory".

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/blame.tcl

@@ -370,6 +370,8 @@ method _history_menu {} {
 
 		if {[regexp {^[0-9a-f]{40}$} $c]} {
 			set t [string range $c 0 8]...
+		} elseif {$c eq {}} {
+			set t {Working Directory}
 		} else {
 			set t $c
 		}