Refactoring: Split up diff_tree_stdin
Refactoring: Split up diff_tree_stdin

Into a first half that determines what operation to do, and a second
half that does it.

Currently the only operation is diffing one or more commits, but a
later patch will add diffing of trees, at which point this refactoring
will pay off.

Signed-off-by: Karl Hasselström <7985ada83f2f009c597f77bfcbafe8daa9e79c7d@treskal.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git