Use a "best effort" strategy in scheduled maintenance

Over in https://github.com/microsoft/git/issues/623, it was pointed out that scheduled maintenance will error out when it encounters a missing repository. The scheduled maintenance should exit with an error, all right, but what about the remaining repositories for which maintenance was scheduled, and that may not be missing?

This patch series addresses this by introducing a new for-each-repo option and then using it in the command that is run via scheduled maintenance.

Changes since v2 (thanks Patrick, Jeff and Junio):

* When not passing the new --keep-going option, the exit code is the same as before.
* Clarified in the documentation of the --keep-going option that it is 0 for success, 1 for failure, no matter the exact exit code of the failing command invocation(s).

Changes since v1 (thanks Eric!):

* Changed the option's documentation to reflect the current state (instead of the original design)
* Fixed grammar issues

Johannes Schindelin (2):
for-each-repo: optionally keep going on an error
maintenance: running maintenance should not stop on errors

 Documentation/git-for-each-repo.txt |  9 +++++++++
 builtin/for-each-repo.c             | 13 +++++++++++--
 builtin/gc.c                        |  7 ++++---
 t/t0068-for-each-repo.sh            | 16 ++++++++++++++++
 t/t7900-maintenance.sh              |  6 +++---
 5 files changed, 43 insertions(+), 8 deletions(-)

base-commit: 3c2a3fdc388747b9eaf4a4a4f2035c1c9ddb26d0
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1719%2Fdscho%2Ffor-each-repo-stop-on-error-2.44-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1719/dscho/for-each-repo-stop-on-error-2.44-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1719

Range-diff vs v2:

1: abd796894c8 ! 1: 39ee6386aab for-each-repo: optionally keep going on an error

     @@ Commit message
          repository, still setting the exit code to indicate an error occurred.
      
          Helped-by: Eric Sunshine <[email protected]>
     +    Helped-by: Patrick Steinhardt <[email protected]>
          Signed-off-by: Johannes Schindelin <[email protected]>
      
       ## Documentation/git-for-each-repo.txt ##
     @@ Documentation/git-for-each-repo.txt: These config values are loaded from system,
      +	Continue with the remaining repositories if the command failed
      +	on a repository. The exit code will still indicate that the
      +	overall operation was not successful.
     +++
     ++Note that the exact exit code of the failing command is not passed
     ++through as the exit code of the `for-each-repo` command: If the command
     ++failed in any of the specified repositories, the overall exit code will
     ++be 1.
       
       SUBPROCESS BEHAVIOR
       -------------------
     @@ builtin/for-each-repo.c: int cmd_for_each_repo(int argc, const char **argv, cons
       
      -	for (i = 0; !result && i < values->nr; i++)
      -		result = run_command_on_repo(values->items[i].string, argc, argv);
     -+	for (i = 0; (keep_going || !result) && i < values->nr; i++)
     -+		if (run_command_on_repo(values->items[i].string, argc, argv))
     ++	for (i = 0; i < values->nr; i++) {
     ++		int ret = run_command_on_repo(values->items[i].string, argc, argv);
     ++		if (ret) {
     ++			if (!keep_going)
     ++					return ret;
      +			result = 1;
     ++		}
     ++	}
       
       	return result;
       }

2: 1ae11553052 = 2: 540962859c5 maintenance: running maintenance should not stop on errors

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