Checksum mismatch when importing a symlink that changes into a regular file

[Max Bowsher]

Affects https://code.edge.launchpad.net/~vcs-imports/kdebase/trunk

FileRevisionBuildEditor._apply_textdelta checks the base chunks retrieved from Bazaar match the base checksum that the Subversion server sends. That's fine, except when the base revision of the file is actually a symlink, in which case the Bazaar representation of its text is "", but the Subversion representation is "link path/to/target". TextChecksumMismatch is raised.