Extend checkout --track with a fetch mode to refresh start-point.

Changes in v15:

 * Reword commit message to lead with motivation.
 * Drop RESOLVE_REF_NO_RECURSE so <ns>/HEAD lookup matches what git checkout
   does for the same ref. Drop redundant check_refname_format on a ref we
   just read. Replace memset with brace initializer. Use refs_ref_exists and
   pass NULL for the OID out-params.
 * Split the "set HEAD" error onto its own line via die_message and advise,
   matching the suggested format.
 * Remove 6 redundant tests, replace test $a = $b with test_cmp_rev, and
   rename test titles to avoid "namespace".

Changes in v14:

 * Handle .h files in a better way.

Changes in v13:

 * Create a preparatory commit that exposes find_tracking_remote_for_ref()
   and advise_ambiguous_fetch_refspec() from branch.c, so checkout can reuse
   the same lookup git branch --track uses.
 * Use advise_ambiguous_fetch_refspec() for the "multiple remotes match"
   case, so the wording matches git branch --track.

Harald Nordgren (2):
  branch: expose helpers for finding the remote owning a tracking ref
  checkout: extend --track with a "fetch" mode to refresh start-point

 Documentation/git-checkout.adoc |  17 ++-
 Documentation/git-switch.adoc   |   5 +-
 branch.c                        |  96 +++++++-------
 branch.h                        |  16 +++
 builtin/checkout.c              | 138 +++++++++++++++++++-
 t/t7201-co.sh                   | 222 ++++++++++++++++++++++++++++++++
 6 files changed, 443 insertions(+), 51 deletions(-)

base-commit: ab776a62a78576513ee121424adb19597fbb7613

Submitted-As: https://lore.kernel.org/git/pull.2281.v15.git.git.1782338098.gitgitgadget@gmail.com
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