merge-file: let conflict markers match end-of-line style of the context

When merging files with CR/LF line endings, the conflict markers should
match those, lest the output file has mixed line endings.

This is particularly of interest on Windows, where some editors get
*really* confused by mixed line endings.

The original version of this patch by Beat Bolli respected core.eol, and
a subsequent improvement by this developer also respected gitattributes.
This approach was suboptimal, though: `git merge-file` was invented as a
drop-in replacement for GNU merge and as such has no problem operating
outside of any repository at all!

Another problem with the original approach was pointed out by Junio
Hamano: legacy repositories might have their text files committed using
CR/LF line endings (and core.eol and the gitattributes would give us a
false impression there). Therefore, the much superior approach is to
simply match the context's line endings, if any.

We actually do not have to look at the *entire* context at all: if the
files are all LF-only, or if they all have CR/LF line endings, it is
sufficient to look at just a *single* line to match that style. And if
the line endings are mixed anyway, it is *still* okay to imitate just a
single line's eol: we will just add to the pile of mixed line endings,
and there is nothing we can do about that.

So what we do is: we look at the line preceding the conflict, falling
back to the line preceding that in case it was the last line and had no
line ending, falling back to the first line, first in the first
post-image, then the second post-image, and finally the pre-image.
If we find consistent CR/LF (or undecided) end-of-line style, we match
that, otherwise we use LF-only line endings for the conflict markers.

Note that while it is true that there have to be at least two lines we
can look at (otherwise there would be no conflict), the same is not true
for line *endings*: the three files in question could all consist of a
single line without any line ending, each. In this case we fall back to
using LF-only.

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

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