Introduce `range-diff` to compare iterations of a topic branch

This command does not do a whole lot so far, apart from showing a usage
that is oddly similar to that of `git tbdiff`. And for a good reason:
the next commits will turn `range-branch` into a full-blown replacement
for `tbdiff`.

At this point, we ignore tbdiff's color options, as they will all be
implemented later using diff_options.

Since f318d739159 (generate-cmds.sh: export all commands to
command-list.h, 2018-05-10), every new command *requires* a man page to
build right away, so let's also add a blank man page, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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