Installer: Use sh.exe directly in Git Bash icon if possible (fixes #71).

Installer: Use sh.exe directly in Git Bash icon if possible (fixes #71).

To avoid side-effects on other cmd.exe windows, point the Git Bash icon
directly to sh.exe on Windows Vista and newer. On 64-bit Windows XP,
launching through 32-bit cmd.exe is required to get sh.exe to launch as
a 32-bit process, but on Windows Vista and newer, this workaround is no
longer required (see commit b6ea386007f0c194bf1e21c057386f639eed831c). On
Windows 8, if a start menu link points to cmd.exe but uses a different
icon, that icon is also used by some other cmd.exe windows, even if the
link has command line parameters. On Windows 7, the side-effect is
different; if the Git Bash link uses cmd.exe, Windows groups Git Bash
together with other cmd.exe windows. By avoiding launching through
cmd.exe when it is not necessary, we avoid affecting the icon used by
other cmd.exe windows.

Signed-off-by: David Matson <gitcoder@outlook.com>