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

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              | 139 +++++++++++++++-
 t/t7201-co.sh                   | 276 ++++++++++++++++++++++++++++++++
 6 files changed, 498 insertions(+), 51 deletions(-)

base-commit: 4621f8ce5e9b97aa2e8d0d9ffe9d25df2471074d

Submitted-As: https://lore.kernel.org/git/pull.2281.v14.git.git.1781786652.gitgitgadget@gmail.com
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