From http://mercurial.selenic.com/wiki/GitExtendedDiffFormat

Mercurial's default format for showing changes between two
versions of a file is compatible with the unified format of GNU
diff, which can be used by GNU patch and many other standard
tools.

While this standard format is often enough, it does not encode the
following information:

• executable status and other permission bits
• copy or rename information
• changes in binary files
• creation or deletion of empty files

Mercurial also supports the extended diff format from the git VCS
which addresses these limitations. The git diff format is not
produced by default because a few widespread tools still do not
understand this format.

Crucible is one of the "few widespread tools" that does not support the extended git diffs.