for-each-repo: run subcommands on configured repos

It can be helpful to store a list of repositories in global or system
config and then iterate Git commands on that list. Create a new builtin
that makes this process simple for experts. We will use this builtin to
run scheduled maintenance on all configured repositories in a future
change.

The test is very simple, but does highlight that the "--" argument is
optional.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

command-list.txt

@@ -94,6 +94,7 @@ git-fetch-pack                          synchingrepositories
 git-filter-branch                       ancillarymanipulators
 git-fmt-merge-msg                       purehelpers
 git-for-each-ref                        plumbinginterrogators
+git-for-each-repo                       plumbinginterrogators
 git-format-patch                        mainporcelain
 git-fsck                                ancillaryinterrogators          complete
 git-gc                                  mainporcelain