In "git log --decorate", you would see the commit header like this:

    commit ... (HEAD, jc/decorate-leaky-separator-color)

where "commit ... (" is painted in color.diff.commit, "HEAD" in
color.decorate.head, ", " in color.diff.commit, the branch name in
color.decorate.branch and then closing ")" in color.diff.commit.

If you wanted to paint the HEAD and local branch name in the same
color as the body text (perhaps because cyan and green are too faint
on a black-on-white terminal to be readable), you would not want to
have to say

    [color "decorate"]
        head = black
        branch = black

because that you would not be able to reuse same configuration on a
white-on-black terminal.  You would naively expect

    [color "decorate"]
        head = normal
	branch = normal

to work, but unfortunately it does not.  It paints the string "HEAD"
and the branch name in the same color as the opening parenthesis or
comma between the decoration elements.  This is because the code
forgets to reset the color after printing the "prefix" in its own
color.

It theoretically is possible that some people were expecting and
relying on that the attribute set as the "diff.commit" color, which
is used to draw these opening parenthesis and inter-item comma, is
inherited by the drawing of branch names, but it is not how the
coloring works everywhere else.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>