rebase -i: rearrange fixup/squash lines using the rebase--helper

This operation has quadratic complexity, which is especially painful
on Windows, where shell scripts are *already* slow (mainly due to the
overhead of the POSIX emulation layer).

Let's reimplement this with linear complexity (using a hash map to
match the commits' subject lines) for the common case; Sadly, the
fixup/squash feature's design neglected performance considerations,
allowing arbitrary prefixes (read: `fixup! hell` will match the
commit subject `hello world`), which means that we are stuck with
quadratic performance in the worst case.

The reimplemented logic also happens to fix a bug where commented-out
lines (representing empty patches) were dropped by the previous code.

While at it, clarify how the fixup/squash feature works in `git rebase
-i`'s man page.

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

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