git-rebase.txt: use back-ticks consistently

While inspecting the 'git rebase' documentation, I noticed that it is
inconsistent with how it uses back-ticks (or other punctuation) for
identifying Git commands, command-line arguments, or values for those
arguments.

Sometimes, an argument (like '--interactive') would appear without any
punctuation, causing the argument to not have any special formatting.
Other times, arguments or 'git rebase' itself would have single-quotes
giving a bold look (in the HTML documentation at least).

By consistently using back-ticks, these types of strings appear in a
monospace font with special highlighting to appear more clearly as text
that exists in a command-line invocation of a Git command.

This rather-large diff is the result of scanning git-rebase.txt and
adding back-ticks as appropriate. Some are adding back-ticks where there
was no punctuation. Others are replacing single quotes.

There are also a few minor cleanups in the process, including those
found by reviewers.

Helped-by: Phillip Wood <[email protected]>
Helped-by: Junio C Hamano <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>