git-apply --whitespace=fix: fix whitespace fuzz introduced by previous run

When you have more than one patch series, an earlier one of which
tries to introduce whitespace breakages and a later one of which
has such a new line in its context, "git-apply --whitespace=fix"
will apply and fix the whitespace breakages in the earlier one,
making the resulting file not to match the context of the later
patch.

A short demonstration is in the new test, t4125.

For example, suppose the first patch is:

    diff a/hello.txt b/hello.txt
    --- a/hello.txt
    +++ b/hello.txt
    @@ -20,3 +20,3 @@
     Hello world.$
    -How Are you$
    -Today?$
    +How are you $
    +today? $

to fix broken case in the string, but it introduces unwanted
trailing whitespaces to the result (pretend you are looking at
"cat -e" output of the patch --- '$' signs are not in the patch
but are shown to make the EOL stand out).  And the second patch
is to change the wording of the greeting further:

    diff a/hello.txt b/hello.txt
    --- a/hello.txt
    +++ b/hello.txt
    @@ -18,5 +18,5 @@
     Greetings $

    -Hello world.$
    +Hello, everybody. $
     How are you $
    -today? $
    +these days? $

If you apply the first one with --whitespace=fix, you will get
this as the result:

    Hello world.$
    How are you$
    today?$

and this does not match the preimage of the second patch, which
demands extra whitespace after "How are you" and "today?".

This series is about teaching "git apply --whitespace=fix" to
cope with this situation better.  If the patch does not apply,
it rewrites the second patch like this and retries:

    diff a/hello.txt b/hello.txt
    --- a/hello.txt
    +++ b/hello.txt
    @@ -18,5 +18,5 @@
     Greetings$

    -Hello world.$
    +Hello, everybody.$
     How are you$
    -today?$
    +these days?$

This is done by rewriting the preimage lines in the hunk
(i.e. the lines that begin with ' ' or '-'), using the same
whitespace fixing rules as it is using to apply the patches, so
that it can notice what it did to the previous ones in the
series.

A careful reader may notice that the first patch in the example
did not touch the "Greetings" line, so the trailing whitespace
that is in the original preimage of the second patch is not from
the series.  Is rewriting this context line a problem?

If you think about it, you will realize that the reason for the
difference is because the submitter's tree was based on an
earlier version of the file that had whitespaces wrong on that
"Greetings" line, and the change that introduced the "Greetings"
line was added independently of this two-patch series to our
tree already with an earlier "git apply --whitespace=fix".

So it may appear this logic is rewriting too much, it is not
so.  It is just rewriting what we would have rewritten in the
past.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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