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>

git-svn.perl

@@ -367,9 +367,9 @@ Git::SVN::init_vars();
 eval {
 	Git::SVN::verify_remotes_sanity();
 	$cmd{$cmd}->[0]->(@ARGV);
+	post_fetch_checkout();
 };
 fatal $@ if $@;
-post_fetch_checkout();
 exit 0;
 
 ####################### primary functions ######################
@@ -1598,8 +1598,8 @@ sub rebase_cmd {
 
 sub post_fetch_checkout {
 	return if $_no_checkout;
+	return if verify_ref('HEAD^0');
 	my $gs = $Git::SVN::_head or return;
-	return if verify_ref('refs/heads/master^0');
 
 	# look for "trunk" ref if it exists
 	my $remote = Git::SVN::read_all_remotes()->{$gs->{repo_id}};
@@ -1612,9 +1612,8 @@ sub post_fetch_checkout {
 		}
 	}
 
-	my $valid_head = verify_ref('HEAD^0');
-	command_noisy(qw(update-ref refs/heads/master), $gs->refname);
-	return if ($valid_head || !verify_ref('HEAD^0'));
+	command_noisy(qw(update-ref HEAD), $gs->refname);
+	return unless verify_ref('HEAD^0');
 
 	return if $ENV{GIT_DIR} !~ m#^(?:.*/)?\.git$#;
 	my $index = $ENV{GIT_INDEX_FILE} || "$ENV{GIT_DIR}/index";