Merge branch 'vl/grep-configurable-threads'

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