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

environment.c

@@ -31,6 +31,7 @@ const char *git_log_output_encoding;
 const char *apply_default_whitespace;
 const char *apply_default_ignorewhitespace;
 const char *git_attributes_file;
+const char *git_hooks_path;
 int zlib_compression_level = Z_BEST_SPEED;
 int core_compression_level;
 int core_compression_seen;