git-gui: Use sh.exe in Cygwin shortcuts

Because we are trying to execute /bin/sh we know it must be a real
Windows executable and thus ends with the standard .exe suffix.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

lib/shortcut.tcl

@@ -54,7 +54,7 @@ proc do_cygwin_shortcut {} {
 				set sh [exec cygpath \
 					--windows \
 					--absolute \
-					/bin/sh]
+					/bin/sh.exe]
 				set me [exec cygpath \
 					--unix \
 					--absolute \