Merge branch 'jc/grep-pcre-loose-ends'

"git log -F -E --grep='<ere>'" failed to use the given <ere>
pattern as extended regular expression, and instead looked for the
string literally.  The early part of this series is a fix for it;
the latter part teaches log to respect the grep.* configuration.

* jc/grep-pcre-loose-ends:
  log: honor grep.* configuration
  log --grep: accept --basic-regexp and --perl-regexp
  log --grep: use the same helper to set -E/-F options as "git grep"
  revisions: initialize revs->grep_filter using grep_init()
  grep: move pattern-type bits support to top-level grep.[ch]
  grep: move the configuration parsing logic to grep.[ch]
  builtin/grep.c: make configuration callback more reusable

Documentation/rev-list-options.txt

@@ -79,6 +79,11 @@ if it is part of the log message.
 
 	Match the regexp limiting patterns without regard to letters case.
 
+--basic-regexp::
+
+	Consider the limiting patterns to be basic regular expressions;
+	this is the default.
+
 -E::
 --extended-regexp::
 
@@ -91,6 +96,11 @@ if it is part of the log message.
 	Consider the limiting patterns to be fixed strings (don't interpret
 	pattern as a regular expression).
 
+--perl-regexp::
+
+	Consider the limiting patterns to be Perl-compatible regexp.
+	Requires libpcre to be compiled in.
+
 --remove-empty::
 
 	Stop when a given path disappears from the tree.