I use a set of small scripts [*1*] directly on top of the core
git, which needed to make patches out of diff-tree and
diff-cache output.  Its output is compatible with what show-diff
produces.

Since this helper program would be generally useful for other
wrapper layer programs like Cogito, I am submitting it in two
parts.  The first part is to rip out the external diff program
interface part out of show-diff.c and moving it into a common
library.  The second part introduces the diff-tree-helper
program.

[Footnotes]

*1* This is found at http://members.cox.net/junkio/