Internally, the `git range-diff` command spawns a `git log` process and
parses its output for the given commit ranges.

This works well when the patches that need to be compared are present in
the local repository in the form of commits.

In scenarios where that is not the case, the `range-diff` command is
currently less helpful.

The Git mailing list is such a scenario: Instead of using Git to
exchange commits, the patches are sent there as plain-text and no commit
range can be specified to let `range-diff` consume those patches.

Instead, the expectation is to download the mails, apply them locally
and then use `range-diff`. This can be quite cumbersome e.g. when a
suitable base revision has to be found first where the patch applies
cleanly.

Let's offer a way to read those patches from pre-prepared MBox files
instead when an argument "mbox:<filename>" is passed instead of a commit
range.

For extra convenience, interpret the filename `-` as standard input.
This makes it easy to compare contributions on the mailing list with the
actual commits that were integrated into Git's main branch. Example:

	commit=5c4003ca3f0e9ac6d3c8aa3e387ff843bd440411
	mid=bdfa3845b81531863941e6a97c28eb1afa62dd2c.1489435755.git.johannes.schindelin@gmx.de
	curl -s https://lore.kernel.org/git/$mid/raw |
	git range-diff mbox:- $commit^!

This addresses gitgitgadget#207

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Undo long header handling, will redo in a bit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Undo in-body handling; will redo in a moment.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The mbox format allows for long header values to be broken apart into
78-column wide chunks. Parse those, so that even long commit titles are
handled correctly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Need to add a test for a bare diff (i.e. without commit message), to
allow things like:

	git range-diff $COMMIT^! mbox:<(git diff HEAD)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>