[PATCH v5 0/3] fetch: skip unnecessary tasks when using --negotiate-only

`git fetch --negotiate-only` is used internally by push negotation and it behaves very differently from other uses of `git fetch`, e.g. it does not update refs or fetch objects. But because of how cmd_fetch() is written, `git fetch --negotiate-only` performs tasks that it shouldn't. This is results in behavior that is unnecessary at best, and incorrect at worst:

* Submodules are updated if enabled by recurse.submodules=true, but
  negotiation fetch doesn't actually update the repo, so this doesn't
  make sense.
* Commit graphs will be written if enabled by
  fetch.writeCommitGraph=true, but this is unnecessary because no
  objects are fetched [1]. 
* gc is run, but according to the commit message in [2], we only do this
  because we expect `git fetch` to introduce objects.

Make `git fetch --negotiate-only` handle these tasks more rigorously by doing the following:

* Make cmd_fetch() skip irrelevant tasks if we know for certain that
objects will not be fetched
* Disable submodule recursion and die() if a user explicitly asks for it

[1] This is also confirmed by Documentation/config/fetch.txt, which
  states that Git should only write commit graphs if a pack-file is
  downloaded.
[2] 131b8fcbfb (fetch: run gc --auto after fetching, 2013-01-26)

Changes since v4:
* drop an unnecessary block (thanks Junio!)

Changes since v3:
* change commit message subject: builtin/fetch -> fetch --negotiate-only
* move the 'goto cleanup' to _after_ the submodule updating task because
  we may want to update submodules even if objects were not fetched (as
  pointed out by Junio, thanks!)
* disable submodule recursion in the patch that checks for
  --negotiate-only + --recurse-submodules, so we never silently ignore
  --recurse-submodules.
* incorporate some of Jonathan's suggestions (thanks!)

Changes since v2:
* added a prepatory patch that introduces a "goto cleanup"
* drop an unnecessary line move (as suggested by Junio)
* check for user-given --recurse-submodules and die() (as suggested by
  Jonathan and Junio)
* update --negotiate-only's documentation

Changes since v1:
* added more context to commit message
* added a NEEDSWORK comment

Glen Choo (3):
  fetch: use goto cleanup in cmd_fetch()
  fetch: skip tasks related to fetching objects
  fetch --negotiate-only: do not update submodules

 Documentation/fetch-options.txt |  1 +
 builtin/fetch.c                 | 40 ++++++++++++++++++++++++++++++---
 t/t5516-fetch-push.sh           | 12 ++++++++++
 t/t5702-protocol-v2.sh          | 12 ++++++++++
 4 files changed, 62 insertions(+), 3 deletions(-)

Range-diff against v4:
1:  ffa1a24109 = 1:  ffa1a24109 fetch: use goto cleanup in cmd_fetch()
2:  b0c73e8135 = 2:  b0c73e8135 fetch: skip tasks related to fetching objects
3:  914d30866f ! 3:  f929297961 fetch --negotiate-only: do not update submodules
    @@ builtin/fetch.c: int cmd_fetch(int argc, const char **argv, const char *prefix)
     +	if (recurse_submodules_cli != RECURSE_SUBMODULES_DEFAULT)
     +		recurse_submodules = recurse_submodules_cli;
     +
    -+	if (negotiate_only) {
    ++	if (negotiate_only)
     +		switch (recurse_submodules_cli) {
     +		case RECURSE_SUBMODULES_OFF:
     +		case RECURSE_SUBMODULES_DEFAULT: {
    @@ builtin/fetch.c: int cmd_fetch(int argc, const char **argv, const char *prefix)
     +		default:
     +			die(_("--negotiate-only and --recurse-submodules cannot be used together"));
     +		}
    -+	}
    ++
     +
      	if (recurse_submodules != RECURSE_SUBMODULES_OFF) {
      		int *sfjc = submodule_fetch_jobs_config == -1

base-commit: 90d242d36e248acfae0033274b524bfa55a947fd