[PATCH 0/6] git-name-rev -d and git-describe improvements

This series of 6 patches improves performance for both describe and `show-ref -d` on repositories with a lot of tags, and finally ends with the display of the current tag for detached HEAD in the bash prompt rather than the abbreviated SHA-1.

The `show-ref -d` improvements should help both gitk and gitweb performance as it reduces the total number of lstats necessary (we were doing an extra unnecessary lstat on each packed ref).

The `describe --exact-match` flag is useful in scripts, as shown by the final patch in the series.

  1) Protect peel_ref fallback case from NULL parse_object result
  2) Optimize peel_ref for the current ref of a for_each_ref callback
  3) Teach git-describe to use peeled ref information when scanning tags
  4) Avoid accessing non-tag refs in git-describe unless --all is requested
  5) Teach git-describe --exact-match to avoid expensive tag searches
  6) Use git-describe --exact-match in bash prompt on detached HEAD

 Documentation/git-describe.txt         |    5 ++++
 builtin-describe.c                     |   35 ++++++++++++++++++++++++-------
 contrib/completion/git-completion.bash |    5 +++-
 refs.c                                 |   27 +++++++++++++++++++-----
 4 files changed, 57 insertions(+), 15 deletions(-)