vcs-svn: add missing cast to printf argument

gcc -m32 correctly warns:

vcs-svn/fast_export.c: In function 'fast_export_commit':
vcs-svn/fast_export.c:54:2: warning: format '%llu' expects
argument of type 'long long unsigned int', but argument 2
has type 'unsigned int' [-Wformat]

Fix it.

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