WinGit: Fix several regressions with the VB helper script

WinGit: Fix several regressions with the VB helper script

The changes in eec84b0 introduced several regressions:

- The window icon is no longer set to the Git icon
- The window title is a random name instead of "Git Bash"
- Changes to the window properties can no longer be saved because the
  dynamically created shortcut is automatically deleted

This commit fixes the above issues while maintaining compatibility with UNC
paths. This is done by:

- Specifying an icon location
- Always using "Git Bash" instead of a random name
- Not automatically deleting the shortcut from the temporary directory

The last two points should not be a problem as "CreateShortCut" does not
overwrite but opens existing shortcuts.

Finally, the code was cleaned-up and simplified.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>