add -p: prefer color.diff.context over color.diff.plain

add -p: prefer color.diff.context over color.diff.plain

Git's diff machinery allows users to override the colors to use in
diffs, even the plain-colored context lines. As of 8dbf3eb6850 (diff.h:
rename DIFF_PLAIN color slot to DIFF_CONTEXT, 2015-05-27), the preferred
name of the config setting is `color.diff.context`, although Git still
allows `color.diff.plain`.

In the context of `git add -p`, this logic is a bit hard to replicate:
`git_diff_basic_config()` reads all config values sequentially and if it
sees _any_ `color.diff.context` or `color.diff.plain`, it accepts the
new color. The Perl version of `git add -p` needs to go through `git
config --get-color`, though, which allows only one key to be specified.
The same goes for the built-in version of `git add -p`, which has to go
through `repo_config_get_value()`.

The best we can do here is to look for `.context` and if none is found,
fall back to looking for `.plain`, and if still not found, fall back to
the hard-coded default (which in this case is simply the empty string,
as context lines are typically rendered without colored).

This still leads to inconsistencies when both config names are used: the
initial diff will be colored by the diff machinery. Once edited by a
user, a hunk has to be re-colored by `git add -p`, though, which would
then use the other setting to color the context lines.

In practice, this is not _all_ that bad. The `git config` manual says
this in the `color.diff.<slot>`:

	`context` (context text - `plain` is a historical synonym)

We should therefore assume that users use either one or the other, but
not both names. Besides, it is relatively uncommon to look at a hunk
after editing it because it is immediately staged by default.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>