gitcha#GitSHAComplete(): Use `complete()` to open popup menu (WIP)

Work in progress

Rough code for a working implementation using `complete()` instead of
'completefunc'.

Rejigger `start` and make a new `base` variable since we can no longer
get it as an argument.

Correctly reset 'completeopt' to its original user value after opening
the popup menu.

Return an empty string from the function as recommended by

    :h complete()

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