harmonize --current-branch short option across subcommands

We're inconsistent with the short option for this flag:
* gitc-init: <none as -c is already used>
* info:      -b
* init:      -c
* overview:  -b
* sync:      -c
* upload:   --cbr

Since info & overview are not as heavily used as the others, switch
them from -b to -c.  We leave -b in as a hidden alias for now.

Similarly, switch upload from --cbr to just -c.  A lot of people
use --cbr, so we leave this as a hidden alias for now too.

Ideally gitc-init wouldn't use -c, but that ship has sailed, and
we're more likely to deprecate gitc entirely at this point.

This provides a consistent set of options across subcommands.

Bug: https://crbug.com/gerrit/12401
Change-Id: Iec249729223866fe1ea0ebabed12ca851cc38b35
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/304902
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>