git-gui: Add Reset to the Branch menu.

cehteh on #git noticed that there was no way to perform a reset --hard
from within git-gui.  When I pointed out this was Merge->Abort Merge
cehteh said this is not very understandable, and that most users would
never guess to try that option unless they were actually in a merge.

So Branch->Reset is now also a way to cause a reset --hard from within
the UI.  Right now the confirmation dialog is the same as the one used
in Merge->Abort Merge.

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

git-gui.sh

@@ -5256,6 +5256,12 @@ if {[is_enabled branch]} {
 		-font font_ui
 	lappend disable_on_lock [list .mbar.branch entryconf \
 		[.mbar.branch index last] -state]
+
+	.mbar.branch add command -label {Reset...} \
+		-command do_reset_hard \
+		-font font_ui
+	lappend disable_on_lock [list .mbar.branch entryconf \
+		[.mbar.branch index last] -state]
 }
 
 # -- Commit Menu