Merge branch 'vl/grep-configurable-threads'

"git grep" can now be configured (or told from the command line)
how many threads to use when searching in the working tree files.

* vl/grep-configurable-threads:
  grep: add --threads=<num> option and grep.threads configuration
  grep: slight refactoring to the code that disables threading
  grep: allow threading even on a single-core machine

Documentation/config.txt

@@ -1450,6 +1450,10 @@ grep.extendedRegexp::
 	option is ignored when the 'grep.patternType' option is set to a value
 	other than 'default'.
 
+grep.threads::
+	Number of grep worker threads to use.
+	See `grep.threads` in linkgit:git-grep[1] for more information.
+
 gpg.program::
 	Use this custom program instead of "gpg" found on $PATH when
 	making or verifying a PGP signature. The program must support the