tests/test-debugindexdot.t

templatekw: introduce showlatesttags() to handle {latesttag} keywords

The keywords {changes}, {distance} and {tag} will be available on a future
template method that will allow pattern matching against tag names.  For
consistency, these should be available on the existing {latesttag} keyword as
well.

I debated whether or not to add {tag} instead of just continuing with the
existing {latesttag}.  But it seems clearer not to have the same name for two
distinct things (a list in the LHS of %, and an individual tag value on the
right).

The value of latesttags[0] is the date of commit for the cset to which the tag
is applied (i.e. not the date the tag was applied), and therefore isn't made
visible because it doesn't seem interesting.  It appears that this is merely an
internal implementation detail for sorting csets in a stable manner when there
are different branches.

Just exercise debugindexdot
Create a short file history including a merge.
  $ hg init t
  $ cd t
  $ echo a > a
  $ hg ci -qAm t1 -d '0 0'
  $ echo a >> a
  $ hg ci -m t2 -d '1 0'
  $ hg up -qC 0
  $ echo b >> a
  $ hg ci -m t3 -d '2 0'
  created new head
  $ HGMERGE=true hg merge -q
  $ hg ci -m merge -d '3 0'

  $ hg debugindexdot .hg/store/data/a.i
  digraph G {
  	-1 -> 0
  	0 -> 1
  	0 -> 2
  	2 -> 3
  	1 -> 3
  }

  $ cd ..