Merge branch 'ab/grep-simplify-extended-expression'

Merge branch 'ab/grep-simplify-extended-expression'

Giving "--invert-grep" and "--all-match" without "--grep" to the
"git log" command resulted in an attempt to access grep pattern
expression structure that has not been allocated, which has been
corrected.

* ab/grep-simplify-extended-expression:
  grep.c: remove "extended" in favor of "pattern_expression", fix segfault