git-gui: Work around odd cygpath bug on Windows.

There appears to be a bug on one of my test systems where cygpath with
the --long-name option is generating a corrupt string that does not
actually refer to sh.exe.  This breaks any desktop icon created by
git-gui as the executable we are trying to invoke does not exist.
Since Cygwin is typically installed as C:\cygwin long path names is
probably not actually necessary to link to the shell.

I also added a small echo to the start of the icon script, as it can
take one of my test systems several seconds to startup git-gui.  This
way the user knows we're starting git-gui, and was politely asked to
wait for the action to complete.

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