revspec: Add 'head' keyword to mean revno -1

[Jari Aalto]

I find often to want to write:

  bzr diff -r 15..head

just to find out that I should have used magic number "-1". Please
add 'head' alias to revspec. This would help people coming from
CVS/SVN/GIT to orientate better to bzr.

[John A Meinel (jameinel)]

well, in cvs/svn/git it is actually known as HEAD, at least I've always seen it capitalized.

At least the file is .git/HEAD and the help for SVN says:

  -r [--revision] arg : ARG (some commands also take ARG1:ARG2 range)
                             A revision argument can be one of:
                                NUMBER revision number
                                "{" DATE "}" revision at start of the date
                                "HEAD" latest in repository
                                "BASE" base rev of item's working copy
                                "COMMITTED" last commit at or before BASE
                                "PREV" revision just before COMMITTED

Also capitalized.

And running "git log head" gives me an error, while "git log HEAD" returns something.

In 'svn' both 'svn log -r HEAD' and 'svn log -r head' work.

I don't really like adding all-caps names, but if this is really about compatibility with other systems, I think we would need HEAD, and can also add "head".

[Jari Aalto (jari-aalto)]

Kindly keep the names in small letters, because the other revspec items use small letters as well
(cf. "bzr help revisionspec"). It's easier to type as well.

The format or revision range -rX..Y is also different from e.g. SVN's -rX:Y, to using
small letters in revspec is fine (similar small deviation from the SVN/GIT).