Hi,
Just saw this in Documentation/git-commit-tree.txt at line 75:
A commit comment is read from stdin (max 999 chars). If a changelog
But from looking at the code it appears there is no such limit. That's the size of the buffer used to read the commit message in, but it should read multiple chunks of that size if you give it more than that.
If I'm not missing something, I'd suggest removing that bit in parens.
hth,
Geoff