Append ampersand to "Target" of lnk files created by do_cygwin_shortcut

Append ampersand to "Target" of lnk files created by do_cygwin_shortcut

The git-gui menu item "Repository | Create Desktop Icon" creates a
shortcut (.lnk file) on the Windows desktop.  The purpose of the
created shortcut is to make it easy for a user to launch git-gui
for a particular repo in the future.

A Windows user would expect to see git gui launch when they click
the shortcut; they would not expect (nor want) to see a cmd window
open and remain open in the background.

msysGit avoids opening a command window altogether when it's Git GUI
shortcut is used.  Ideally, git on cygwin would also have shortcuts
that simply open the GUI, but as a first step, this change allows
the shell window to politely disappear after starting git gui as a
background process.

Signed-off-by: Phil Lawrence <prlawrence@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>