completion: match ctags symbol names in grep patterns

completion: match ctags symbol names in grep patterns

A common thing to grep for is the name of a symbol. This
patch teaches the completion for "git grep" to look in
a 'tags' file, if present, to complete a pattern. For
example, in git.git:

  $ make tags
  $ git grep get_sha1<Tab><Tab>
  get_sha1                 get_sha1_oneline
  get_sha1_1               get_sha1_with_context
  get_sha1_basic           get_sha1_with_context_1
  get_sha1_hex             get_sha1_with_mode
  get_sha1_hex_segment     get_sha1_with_mode_1
  get_sha1_mb

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>