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>