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

Documentation/git-grep.txt

@@ -23,6 +23,7 @@ SYNOPSIS
 	   [--break] [--heading] [-p | --show-function]
 	   [-A <post-context>] [-B <pre-context>] [-C <context>]
 	   [-W | --function-context]
+	   [--threads <num>]
 	   [-f <file>] [-e] <pattern>
 	   [--and|--or|--not|(|)|-e <pattern>...]
 	   [ [--[no-]exclude-standard] [--cached | --no-index | --untracked] | <tree>...]
@@ -53,6 +54,10 @@ 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.  If unset (or set to 0),
+	8 threads are used by default (for now).
+
 grep.fullName::
 	If set to true, enable '--full-name' option by default.
 
@@ -227,6 +232,10 @@ OPTIONS
 	effectively showing the whole function in which the match was
 	found.
 
+--threads <num>::
+	Number of grep worker threads to use.
+	See `grep.threads` in 'CONFIGURATION' for more information.
+
 -f <file>::
 	Read patterns from <file>, one per line.