Merge branch 'nd/log-graph-configurable-colors'

Merge branch 'nd/log-graph-configurable-colors'

Some people feel the default set of colors used by "git log --graph"
rather limiting.  A mechanism to customize the set of colors has
been introduced.

* nd/log-graph-configurable-colors:
  document behavior of empty color name
  color_parse_mem: allow empty color spec
  log --graph: customize the graph lines with config log.graphColors
  color.c: trim leading spaces in color_parse_mem()
  color.c: fix color_parse_mem() with value_len == 0