Merge branch 'cc/reset-keep'

* cc/reset-keep:
  Documentation: improve description of "git reset --keep"
  reset: disallow using --keep when there are unmerged entries
  reset: disallow "reset --keep" outside a work tree
  Documentation: reset: describe new "--keep" option
  reset: add test cases for "--keep" option
  reset: add option "--keep" to "git reset"

diff --cc builtin/reset.c
Simple merge