Merge branch 'ab/hooks'

A new configuration variable core.hooksPath allows customizing
where the hook directory is.

* ab/hooks:
hooks: allow customizing where the hook directory is
githooks.txt: minor improvements to the grammar & phrasing
githooks.txt: amend dangerous advice about 'update' hook ACL
githooks.txt: improve the intro section

diff --cc Documentation/config.txt
Simple merge

diff --cc cache.h
Simple merge

diff --cc config.c
Simple merge

diff --cc environment.c
Simple merge

diff --cc run-command.c
Simple merge