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>