git-submodule.sh: support 'checkout' as a valid update mode

According to "Documentation/gitmodules.txt", 'checkout' is a valid
mode of 'submodule.<name>.update'.  Also "git-submodule.sh" refers to
it and processes it correctly.

Update the codepath introduced by ac1fbbda (submodule: do not copy
unknown update mode from .gitmodules, 2013-12-02) to support this
syntax and also validate the value during the 'update' command.

Signed-off-by: Francesco Pretto <ceztko@gmail.com>
Signed-off-by: Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'fp/submodule-checkout-mode' into pu

* fp/submodule-checkout-mode:
  git-submodule.sh: support 'checkout' as a valid update mode
### Reported/known to be broken
Merge branch 'mo/subtree-split-updates' into pu

Comments?

* mo/subtree-split-updates:
  subtree: add --edit option
  subtree: allow --squash and --message with push
  subtree: support split --rejoin --squash
Merge branch 'jc/graph-post-root-gap' into pu

This was primarily a RFH ($gmane/239580).

* jc/graph-post-root-gap:
  WIP: document what we want at the end
  graph: remove unused code a bit
  graph: stuff the current commit into graph->columns[]
Merge branch 'fc/transport-helper-fixes' into pu

Updates transport-helper, fast-import and fast-export to allow the
ref mapping and ref deletion in a way similar to the natively
supported transports.

Reported to break t5541.
Will hold.

* fc/transport-helper-fixes:
  remote-bzr: support the new 'force' option
  test-hg.sh: tests are now expected to pass
  transport-helper: check for 'forced update' message
  transport-helper: add 'force' to 'export' helpers
  transport-helper: don't update refs in dry-run
  transport-helper: mismerge fix

# Conflicts:
# 	Documentation/gitremote-helpers.txt
Merge branch 'fc/completion' into pu

SZEDER Gábor noticed that this breaks "git -c var=val alias" and
also suggested a better description of the change.

Will hold.

* fc/completion:
  completion: fix completion of certain aliases
Merge branch 'ap/remote-hg-skip-null-bookmarks' into pu

Reported to break tests ($gmane/240005)

* ap/remote-hg-skip-null-bookmarks:
  remote-hg: do not fail on invalid bookmarks