for-each-repo: optionally keep going on an error

for-each-repo: optionally keep going on an error

In https://github.com/microsoft/git/issues/623, it was reported that
the regularly scheduled maintenance stops if one repo in the middle of
the list was found to be missing.

This is undesirable, and points out a gap in the design of `git
for-each-repo`: We need a mode where that command does not stop on an
error, but continues to try running the specified command with the other
repositories.

Imitating the `--keep-going` option of GNU make, this commit teaches
`for-each-repo` the same trick: to continue with the operation on all
the remaining repositories in case there was a problem with one
repository, still setting the exit code to indicate an error occurred.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>