diff-highlight: detect --graph by indent

diff-highlight: detect --graph by indent

This patch fixes a corner case where diff-highlight may
scramble some diffs when combined with --graph.

Commit 7e4ffb4c17 (diff-highlight: add support for --graph
output, 2016-08-29) taught diff-highlight to skip past the
graph characters at the start of each line with this regex:

  ($COLOR?\|$COLOR?\s+)*

I.e., any series of pipes separated by and followed by
arbitrary whitespace.  We need to match more than just a
single space because the commit in question may be indented
to accommodate other parts of the graph drawing. E.g.:

 * commit 1234abcd
 | ...
 | diff --git ...

has only a single space, but for the last commit before a
fork:

 | | |
 | * | commit 1234abcd
 | |/  ...
 | |   diff --git

the diff lines have more spaces between the pipes and the
start of the diff.

However, when we soak up all of those spaces with the
$GRAPH regex, we may accidentally include the leading space
for a context line. That means we may consider the actual
contents of a context line as part of the diff syntax. In
other words, something like this:

   normal context line
  -old line
  +new line
   -this is a context line with a leading dash

would cause us to see that final context line as a removal
line, and we'd end up showing the hunk in the wrong order:

  normal context line
  -old line
   -this is a context line with a leading dash
  +new line

Instead, let's a be a little more clever about parsing the
graph. We'll look for the actual "*" line that marks the
start of a commit, and record the indentation we see there.
Then we can skip past that indentation when checking whether
the line is a hunk header, removal, addition, etc.

There is one tricky thing: the indentation in bytes may be
different for various lines of the graph due to coloring.
E.g., the "*" on a commit line is generally shown without
color, but on the actual diff lines, it will be replaced
with a colorized "|" character, adding several bytes. We
work around this here by counting "visible" bytes. This is
unfortunately a bit more expensive, making us about twice as
slow to handle --graph output. But since this is meant to be
used interactively anyway, it's tolerably fast (and the
non-graph case is unaffected).

One alternative would be to search for hunk header lines and
use their indentation (since they'd have the same colors as
the diff lines which follow). But that just opens up
different corner cases. If we see:

  | |    @@ 1,2 1,3 @@

we cannot know if this is a real diff that has been
indented due to the graph, or if it's a context line that
happens to look like a diff header. We can only be sure of
the indent on the "*" lines, since we know those don't
contain arbitrary data (technically the user could include a
bunch of extra indentation via --format, but that's rare
enough to disregard).

Reported-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>