[PATCH 3/3] 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>
---
It's debatable whether this belongs in the generic completion code, as
it really only works if your project uses ctags. But I find it to be a
huge timesaver for finding callsites of functions, especially when
coupled with "git jump grep" from the previous patch.

Undoubtedly you can do something similar with cscope, or with an editor plugin, but using grep feels very natural and simple to me.

For using with "git jump grep", I do this:

  # much easier to type
  git config --global alias.vgrep 'jump grep'

  # and set up completion for it, too
  cat >>~/.bashrc
  _git_vgrep() {
          _git_grep
  }

 contrib/completion/git-completion.bash |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)