submodule update: continue when a checkout fails

submodule update: continue when a checkout fails

"git submodule update" stops at the first error and gives control
back to the user. Only after the user fixes the problematic
submodule and runs "git submodule update" again, the second error
is found. And the user needs to repeat until all the problems are
found and fixed one by one. This is tedious.

Instead, the command can remember which submodules it had trouble with,
continue updating the ones it can, and report which ones had errors at
the end. The user can run "git submodule update", find all the ones that
need minor fixing (e.g. working tree was dirty) to fix them in a single
pass. Then another "git submodule update" can be run to update all.

Note that the problematic submodules are skipped only when they are to
be integrated with a safer value of submodule.<name>.update option,
namely "checkout". Fixing a failure in a submodule that uses "rebase" or
"merge" may need an involved conflict resolution by the user, and
leaving too many submodules in states that need resolution would not
reduce the mental burden on the user.

Signed-off-by: Fredrik Gustafsson <iveqy@iveqy.com>
Mentored-by: Jens Lehmann <Jens.Lehmann@web.de>
Mentored-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>