Tags in components

Nested collections have some issues with tags:

tags in components vs tags in the product problem: "stable" in gcc can’t be different than "stable" in the product.

solution: all components must have a BitKeeper/etc/config var
called tag_prefix that is automagically prepended.  It is set
on attach and defaults to the basename of the component and
attach will fail if that name is already taken in this product.

:TAG: becomes <prefix>.<tag>

even if tag_prefix is not defined.

The prefix may be different in different products, we don't
really care about it other than maybe some work to make sure
that if we pull a change to the prefix we check that it is
still unique.

tagging a sparse collection ("sparse tags") a) At least for tags, and ideally for all csets, we record what was here. b) bk clone --identical means give me what was in that tree as of the implied or specified rev. So if I tag (or if wayne does his magic, commit) in a sparse tree and then clone --identical, the HERE file is the same (modulo aliases vs rootkeys) in the clone as it was in the tree when the commit happened. --identical is mutually exclusive with -s. c) -s@TAG (or likely any rev w/ wayne magic) means the HERE file as of that tag or rev. In fact, bk clone --identical is really just bk clone -s@<implied|specified rev>
```
bk clone --identical		== bk clone -s@+
bk clone --identical -rTAG	== bk clone -rTAG -s@TAG
```
```
To get the subset as of any rev/tag do:
```
```
bk here set @TAG
bk here set @<rev>
```

nested 1.0 requirements: - implement :TAG: - block tags in components until this is complete. - sparse tags can come later - figure out the bk tag syntax cd gcc bk tag gcc.stable # works, has the right prefix bk tag stable # either is a product tag or is an error perhaps bk tag ./stable # means tag this component bk changes -r./stable # means gcc.stable bk -A tag ./stable # do a tag in everything (for detach) # makes ./<tag> pretty compelling