cmd: rename *.C to *.c

These were *.C back in 2003, before the 9 script, to avoid conflicts
with the system utilities. A later change renamed them, but that
change seems to have been lost during the hg->git conversion
because I ran the conversion on a case-insensitive file system.

Change-Id: Id32c99cb9571ef0e185c3cc9e8c8d6d5b48ca195
codereview: use 'git branch -v' for pending

Change-Id: Ib80b19cf69a15860315b2d7a38baf3b05d693acc