gitk: Speed up the reading of references

gitk: Speed up the reading of references

We were doing two execs for each tag - one to map the tag ID to a
commit ID and one to read the contents of the tag for later display.
This speeds up the process by not reading the contents of the tag
(instead it is read later if needed), and by using the -d flag to
git show-ref, which gives us refs/tags/foo^{} lines which give us
the commit ID.  Also this uses string operations instead of regexps.

Signed-off-by: Paul Mackerras <paulus@samba.org>