t4026: test "normal" color

If the user specifies "normal" for a foreground color, this
should be a noop (while this may sound useless, it is the
only way to specify an unchanged foreground color followed
by a specific background color).

We also check that color "-1" does the same thing. This is
not documented, but has worked forever, so let's make sure
we keep supporting it.
parse_color: refactor color storage

When we parse a color name like "red" into its ANSI color
value, we pack the storage into a single int that may take
on many values:

  1. If it's "-2", no value has been specified.

  2. If it's "-1", the value is "normal" (i.e., no color).

  3. If it's 0 through 7, the value is a standard ANSI
     color.

  4. If it's larger (up to 255), it is a 256-color extended
     value.

Given these magic numbers, it is often hard to see what is
going on in the code. Let's refactor this into a struct with
a flag that tells which scheme we are using, along with a
numeric value. This is more verbose, but should hopefully be
simpler to follow. It will also allow us to easily add
support for more schemes, like 24-bit RGB values.

The result is also slightly less efficient to store, but
that's OK; we only store this intermediate state during the
parse, after which we write out the actual ANSI bytes.
parse_color: recognize "no$foo" to clear the $foo attribute

You can turn on ANSI text attributes like "reverse" by
putting "reverse" in your color spec. However, you cannot
ask to turn reverse off.

For common cases, this does not matter. You would turn on
"reverse" at the start of a colored section, and then clear
all attributes with a "reset". However, you may wish to turn
on some attributes, then selectively disable others. For
example:

  git log --format="%C(bold ul yellow)%h%C(noul) %s"

underlines just the hash, but without the need to re-specify
the rest of the attributes. This can also help third-party
programs, like contrib/diff-highlight, that want to turn
some attribute on/off without disrupting existing coloring.

Note that some attribute specifications are probably
nonsensical (e.g., "bold nobold"). We do not bother to flag
such constructs, and instead let the terminal sort it out.
diff-highlight: allow configurable colors

Until now, the highlighting colors were hard-coded in the
script (as "reverse" and "noreverse"), and you had to edit
the script to change them. This patch teaches diff-highlight
to read from color.diff-highlight.* to set them.

In addition, it expands the possiblities considerably by
adding two features:

  1. Old/new lines can be colored independently (so you can
     use a color scheme that complements existing line
     coloring).

  2. Normal, unhighlighted parts of the lines can be colored,
     too. Technically this can be done by separately
     configuring color.diff.old/new and matching it to your
     diff-highlight colors. But you may want a different
     look for your highlighted diffs versus your regular
     diffs.