git-svn: don't create master if another head exists

git-svn: don't create master if another head exists

git-svn insists on creating the "master" head (unless it exists) on every
"fetch". It is useful that it gets created initially, when no head exists
- users expect this git convention of having a "master" branch on initial
clone.

However creating it when there already is another head does not provide any
value - the ref is never updated, so it just gets stale after a while.  Also,
some users find it annoying that it gets recreated, especially when they would
like the git branch names to follow SVN repository branch names. More
background in http://thread.gmane.org/gmane.comp.version-control.git/115030

Make git-svn skip the "master" creation if HEAD already points at a valid head.
This means "master" does get created on initial "clone" but does not get
recreated once a user deletes it.

Also, make post_fetch_checkout work with any head that is pointed to by HEAD,
not just "master".

Also, use fatal error handling consistent with the rest of the program for
post_fetch_checkout.

Signed-off-by: Marcin Owsiany <marcin@owsiany.pl>
Signed-off-by: Eric Wong <normalperson@yhbt.net>