sequencer: introduce the `merge` command

This patch is part of the effort to reimplement `--preserve-merges` with
a substantially improved design, a design that has been developed in the
Git for Windows project to maintain the dozens of Windows-specific patch
series on top of upstream Git.

The previous patch implemented the `label` and `reset` commands to label
commits and to reset to labeled commits. This patch adds the `merge`
command, with the following syntax:

	merge [-C <commit>] <rev> # <oneline>

The <commit> parameter in this instance is the *original* merge commit,
whose author and message will be used for the merge commit that is about
to be created.

The <rev> parameter refers to the (possibly rewritten) revision to
merge. Let's see an example of a todo list (the initial `label onto`
command is an auto-generated convenience so that the label `onto` can be
used to refer to the revision onto which we rebase):

	label onto

	# Branch abc
	reset onto
	pick deadbeef Hello, world!
	label abc

	reset onto
	pick cafecafe And now for something completely different
	merge -C baaabaaa abc # Merge the branch 'abc' into master

To edit the merge commit's message (a "reword" for merges, if you will),
use `-c` (lower-case) instead of `-C`; this convention was borrowed from
`git commit` that also supports `-c` and `-C` with similar meanings.

To create *new* merges, i.e. without copying the commit message from an
existing commit, simply omit the `-C <commit>` parameter (which will
open an editor for the merge message):

	merge abc

This comes in handy when splitting a branch into two or more branches.

Note: this patch only adds support for recursive merges, to keep things
simple. Support for octopus merges will be added later in a separate
patch series, support for merges using strategies other than the
recursive merge is left for the future.

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

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