vcs-svn: add a comment before each commit

vcs-svn: add a comment before each commit

Current svn-fe produces output like this:

	blob
	mark :7382321
	data 5
	hello

	blob
	mark :7382322
	data 5
	Hello

	commit
	mark :3
[...]
	M 100644 :7382321 hello.c
	M 100644 :7382322 hello2.c

This means svn-fe has to keep track of the paths modified in each
commit and the corresponding marks, instead of dealing with each file
as it arrives in input and then forgetting about it.  A better
strategy would be to use inline blobs:

	commit
	mark :3
[...]
	M 100644 inline hello.c
	data 5
	hello
[...]

As a first step towards that, teach svn-fe to notice when the
collection of blobs for each commit starts and write a comment
("# commit 3.") there.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>