vcs-svn: handle log message with embedded NUL

Pass the log message by strbuf instead of as a C-style string and use
fwrite instead of printf to write it to fast-import so embedded '\0'
bytes can be preserved.

Currently "git log" doesn't show the embedded NULs but "git cat-file
commit" can.

While at it, stop including system headers from repo_tree.h.  git
source files need to include git-compat-util.h (or cache.h or
builtin.h) sooner to ensure the appropriate feature test macros are
defined.

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

vcs-svn/svndump.c

@@ -274,7 +274,7 @@ static void handle_revision(void)
 {
 	if (rev_ctx.revision)
 		repo_commit(rev_ctx.revision, rev_ctx.author.buf,
-			rev_ctx.log.buf, dump_ctx.uuid.buf, dump_ctx.url.buf,
+			&rev_ctx.log, dump_ctx.uuid.buf, dump_ctx.url.buf,
 			rev_ctx.timestamp);
 }