Summary:
Rumor has it that sync-all is slowly on the way out. Now that all
subrepositories have been turned into git submodules, sync-all might
not be needed anymore. Nevertheless, here are some changes I had made
while trying to understand why it existed in the first place:

* update comments + help text
* rename some variables for maintainability
    * s/branch_name/remote_name/
    origin is the name of a remote, not a branch

    * s/repo_base/remote_root/
    the word *remote* is key here

    * s/defaultrepo/default_root/
    this was a darcsism, and it doesn't refer to a repository but to the
    root directory of all repositories
* small tweaks
* .git can be a file nowadays
* don't skip END actions on exceptions #8886
  reverts d523f9b3

Test Plan:
Why revert d523f9b3?

I put an old haddock repository from
http://darcs.haskell.org/haddock2.git back in my tree. Now, when running
`sync...