Update the vimscript code for restoring cursor position

Update the vimscript code for restoring cursor position

Using xxd(1) to filter and edit binary files causes the input files
to have dual nature, so to speak, which effectively makes restoring
the cursor position broken.  Fix that by ignoring the "xxd" file type
in the code that restores the cursor position.

Interactive rebasing in git causes files to be edited in vim, which,
similarly to commit messages, are rarely the same as the last one
edited.  Thus, also add "gitrebase" to the list of file types for
which the cursor position isn't restored.

While there, refactor the code a bit to possibly save a few CPU cycles
and to keep the line lengths in check, and use the long form of the
commands and variables, to make the code slightly more consistent and
more understandable to newcomers.

Update the relevant comments in the code and the associated parts of
the documentation, to keep them in sync with the updated code.

Remove some redundant trailing whitespace as well, as spotted.