sync-all: cleanup

+ Remove dead code from the DarcsAges
+ Tweak `git config` flags
    * remove --local, it is not a valid flag
    * --get was used in some places, not in others
+ Simplify reading current branch
+ Delete duplicated code in help
sync-all: delete dead code calling gitInitSubmodules

When running './sync-all get', 'git submodule init' has to be (and is
always) executed, since .gitmodules might contain new submodules (see
commit c3db2b). As a result, after cloning the ghc repo and running the
initial './sync-all get', .git/config will always contain some submodule
sections. Therefore it is not needed to check for this on every
subsequent get or pull, and this code can be deleted.
sync-all: die for real when required repo is missing

Since commit 0e17d4, the default "tag" is "-", not "". It is never
undefined.
sync-all: make --no-dph work for all subcommands
sync-all: set and check variable $repo_is_submodule

Not only does this make the code easier to read, it also fixes a bug.

Starting with commits 691c8a and ccce9f, certain subcommands should
behave differently for submodules. This was done by checking, for each
such subcommand:

    $remotepath eq "-"

This commit corrects that to the check:

    $$line{"remotepath"} eq "-".

Because when we have a clone of a local mirror (checked_out_tree=1),
remotepath actually gets set to $$line{"localpath"}.
sync-all: infer remotepath from .gitmodules file

After this commit, running `sync-all remote set-url` works properly for the
haddock package:

    command: ./sync-all -r git://git.haskell.org remote set-url origin
    before: git://git.haskell.org/packages/haddock.git
    after: git://git.haskell.org/haddock.git

By doing the `remotepath` lookup before the `$is_github_repo` check,
running `sync-all remote set-url` now also works properly for submodule
repos on github:

    command: ./sync-all -r git://github.com/ghc remote set-url origin
    before: git://github.com/ghc/packages/binary
    after: git://github.com/ghc/packages-binary

* Relevant prior commits:
4f4357 "Make `sync-all remote set-url` use normalized `/packages/` urls"
34b072 "Convert haddock into a proper submodule (re #8545)"
974a97 "sync-all: Apply submodule url rewriting also to stuff in util/"
Merge Thomas Miedema’s syn-all improvments

as submitted on #9212.
Fix sync-all get from a local working copy
Fix “Checking for old .. repo” messages