git-svn: clarify the referent of dcommit's optional argument
The documentation of the dcommit subcommand is reworded to clarify that
the optional argument refers to a git branch, not an SVN branch.
The discussion of the optional argument is put into its own paragraph
as is the guidance about using 'dcommit' in preference to 'set-tree'.
The section on REBASE vs. PULL/MERGE is reworded to incorporate the
advice to prefer 'git rebase' previously in the description of 'dcommit'.
Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>