git-gui: Restore the all important shebang line.

Accidentally removed by an unnoticed fat finger accident in vi during
commit 1461c5f3.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/git-gui b/git-gui
index a2a76c1..3f0424a 100755
--- a/git-gui
+++ b/git-gui
@@ -1,3 +1,4 @@
+#!/bin/sh
 # Tcl ignores the next line -*- tcl -*- \
 exec wish "$0" -- "$@"