Match addgitdiffs commit hash specification in the Command-line client to git commit hash specificat

The addgitdiffs command uses Git commit hash syntax identical to the git tool itself except in one regard: specifying individual commit hashes for addgitdiffs is handled the opposite of how the git tool does it.

In Git syntax the following lines are identical:

git diff AAAAAAA..BBBBBBB

git diff AAAAAAA  BBBBBBB

In the Collaborator command-line client the following lines have exactly opposite meanings:

ccollab addgitdiffs ask AAAAAAA..BBBBBBB

ccollab addgitdiffs ask AAAAAAA  BBBBBBB

This seems to be because the addgitdiffs command handles the second case as a list of individual commits, which it assumes are in reverse chronological order (newest to oldest), rather than a direct pass-thru to the git tool (oldest-first) like the first command.  To resolve this, split the addgitdiffs command for specifying a list of commits into a separate command (or require extra options).