Add fallback section for delayed expansion unavailability

Taking cue from start-ssh-agent.cmd, enhanced gitk.cmd to fallback to
the immediate expansion script if delayed expansion is unavailable.

Both script sections are the same, except for the different environment
variable delimiters. When new changes are made, both sections must be
updated correctly, taking care to use the correct delimiter (! or %) to
demarcate the environment variables. (In fact, the fallback section is
exactly the same as the original script before previous commit.)

Signed-off-by: ADTC <send2adtc@gmail.com>