Merge branch 'mk/grep-pcre'

* mk/grep-pcre:
  git-grep: Fix problems with recently added tests
  git-grep: Update tests (mainly for -P)
  Makefile: Pass USE_LIBPCRE down in GIT-BUILD-OPTIONS
  git-grep: update tests now regexp type is "last one wins"
  git-grep: do not die upon -F/-P when grep.extendedRegexp is set.
  git-grep: Bail out when -P is used with -F or -E
  grep: Add basic tests
  configure: Check for libpcre
  git-grep: Learn PCRE
  grep: Extract compile_regexp_failed() from compile_regexp()
  grep: Fix a typo in a comment
  grep: Put calls to fixmatch() and regmatch() into patmatch()
  contrib/completion: --line-number to git grep
  Documentation: Add --line-number to git-grep synopsis

Documentation/git-grep.txt

@@ -12,7 +12,8 @@ SYNOPSIS
 'git grep' [-a | --text] [-I] [-i | --ignore-case] [-w | --word-regexp]
 	   [-v | --invert-match] [-h|-H] [--full-name]
 	   [-E | --extended-regexp] [-G | --basic-regexp]
-	   [-F | --fixed-strings] [-n]
+	   [-P | --perl-regexp]
+	   [-F | --fixed-strings] [-n | --line-number]
 	   [-l | --files-with-matches] [-L | --files-without-match]
 	   [(-O | --open-files-in-pager) [<pager>]]
 	   [-z | --null]
@@ -97,6 +98,11 @@ OPTIONS
 	Use POSIX extended/basic regexp for patterns.  Default
 	is to use basic regexp.
 
+-P::
+--perl-regexp::
+	Use Perl-compatible regexp for patterns. Requires libpcre to be
+	compiled in.
+
 -F::
 --fixed-strings::
 	Use fixed strings for patterns (don't interpret pattern