Git Bash: Set the HOME environment variable if unset

By setting if already here it gets picked up by upstream MSYS's
/etc/profile. Also, setting it dynamically instead of at installation time
solves the problem when users are running an installation done by an
admin, see [1] for details.

[1] https://github.com/msysgit/git/issues/119

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Git Bash: Reorder code a bit to clarify the workflow

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Rename START_DIR to the more approproate LOGIN_DIR

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>