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

diff --cc Documentation/config.txt
Simple merge

diff --cc builtin/grep.c
Simple merge

diff --cc contrib/completion/git-completion.bash
Simple merge