git-svn: add 'log' command, a facsimile of basic `svn log'

git-svn: add 'log' command, a facsimile of basic `svn log'

This quick feature should make it easy to look up svn log
messages when svn users refer to -r/--revision numbers.

The following features from `svn log' are supported:

  --revision=<n>[:<n>] - is supported, non-numeric args are not:
			 HEAD, NEXT, BASE, PREV, etc ...
  -v/--verbose         - just maps to --raw (in git log), so
			 it's completely incompatible with
			 the --verbose output in svn log
  --limit=<n>          - is NOT the same as --max-count,
			 doesn't count merged/excluded commits
  --incremental        - supported (trivial :P)

New features:

  --show-commit        - shows the git commit sha1, as well
  --oneline            - our version of --pretty=oneline

Any other arguments are passed directly to `git log'

Signed-off-by: Eric Wong <normalperson@yhbt.net>