pass config slots as pointers instead of offsets

Many config-parsing helpers, like parse_branch_color_slot,
take the name of a config variable and an offset to the
"slot" name (e.g., "color.branch.plain" is passed along with
"13" to effectively pass "plain"). This is leftover from the
time that these functions would die() on error, and would
want the full variable name for error reporting.

These days they do not use the full variable name at all.
Passing a single pointer to the slot name is more natural,
and lets us more easily adjust the callers to use skip_prefix
to avoid manually writing offset numbers.

This is effectively a continuation of 9e1a5eb, which did the
same for parse_diff_color_slot. This patch covers all of the
remaining similar constructs.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
color_parse: do not mention variable name in error message

Originally the color-parsing function was used only for
config variables. It made sense to pass the variable name so
that the die() message could be something like:

  $ git -c color.branch.plain=bogus branch
  fatal: bad color value 'bogus' for variable 'color.branch.plain'

These days we call it in other contexts, and the resulting
error messages are a little confusing:

  $ git log --pretty='%C(bogus)'
  fatal: bad color value 'bogus' for variable '--pretty format'

  $ git config --get-color foo.bar bogus
  fatal: bad color value 'bogus' for variable 'command line'

This patch teaches color_parse to complain only about the
value, and then return an error code. Config callers can
then propagate that up to the config parser, which mentions
the variable name. Other callers can provide a custom
message. After this patch these three cases now look like:

  $ git -c color.branch.plain=bogus branch
  error: invalid color value: bogus
  fatal: unable to parse 'color.branch.plain' from command-line config

  $ git log --pretty='%C(bogus)'
  error: invalid color value: bogus
  fatal: unable to parse --pretty format

  $ git config --get-color foo.bar bogus
  error: invalid color value: bogus
  fatal: unable to parse default color value

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>