diff.c: introduce emit_diff_symbol

In a later patch we want to buffer all output before emitting it as a
new feature ("markup moved lines") conceptually cannot be implemented
in a single pass over the output.

There are different approaches to buffer all output such as:
* Buffering on the char level, i.e. we'd have a char[] which would
  grow at approximately 80 characters a line. This would keep the
  output completely unstructured, but might be very easy to implement,
  such as redirecting all output to a temporary file and working off
  that. The later passes over the buffer are quite complicated though,
  because we have to parse back any output and then decide if it should
  be modified.

* Buffer on a line level. As the output is mostly line oriented already,
  this would make sense, but it still is a bit awkward as we'd have to
  make sense of it again by looking at the first characters of a line
  to decide what part of a diff a line is.

* Buffer semantically. Imagine there is a formal grammar for the diff
  output and we'd keep the symbols of this grammar around. This keeps
  the highest level of structure in the buffered data, such that the
  actual memory requirements are less than say the first option. Instead
  of buffering the characters of the line, we'll buffer what we intend
  to do plus additional information for the specifics. An output of

    diff --git a/new.txt b/new.txt
    index fa69b07..412428c 100644
    Binary files a/new.txt and b/new.txt differ

  could be buffered as
     DIFF_SYMBOL_DIFF_START + new.txt
     DIFF_SYMBOL_INDEX_MODE + fa69b07 412428c "non-executable" flag
     DIFF_SYMBOL_BINARY_FILES + new.txt

This and the following patches introduce the third option of buffering
by first moving any output to emit_diff_symbol, and then introducing the
buffering in this function.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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