vcs-svn: use constcmp instead of prefixcmp

vcs-svn: use constcmp instead of prefixcmp

Since the length of t is already known, we can simplify a little by
using memcmp() instead of strncmp() to carry out a prefix comparison.
All nearby code already does this.

Noticed in the standalone svn-dump-fast-export project which has not
needed to implement prefixcmp() yet.

Signed-off-by: David Barr <redacted>
Signed-off-by: Jonathan Nieder <redacted>