[PATCH 0/8] More sensible checkout/switch/restore code refactoring

In the beginning, there was only 'git checkout'. We then introduced 'git switch' and 'git restore', borrowing as much original code as possible from the implementation of 'git checkout'.

Unfortunately, this has left the code in a strange shape. Had we started from separate implementations for 'switch' and 'restore' then later merged them into 'checkout', we would not have arrived at a design where three front-end routines, cmd_checkout(), cmd_switch(), and cmd_restore(), funnel into a single monolithic choke point in checkout_main(), which then determines whether we are checking out a branch or paths, and dispatches to checkout_branch() or checkout_paths().

This series is an attempt to repartition the code into a better shape. Because checkout_branch() and checkout_paths() already exist as two reusable helper functions, with sufficient refactoring of checkout_main(), the two newer entry points, cmd_switch() and cmd_restore(), can parse command-line arguments and configuration settings and jump straight into checkout_branch() or checkout_paths() respectively, without worrying about the other path.

Of course, cmd_checkout() still needs to decide whether to check out a branch or a set of paths and dispatch between these two backends.

The series begins with three fairly isolated cleanups that are worth making on their own, even if we do not follow through with the rest of the series. Patches 4 and 5 then refactor various utility functions out of existing code paths, and patch 6 uses them to rewrite the three top-level command entry points to call these functions without going through checkout_main(). Patch 7 is a style-only fix done after the dust settles, which was deliberately kept separate to simplify review.

The final step moves one helper utility out of builtin/checkout.c to top-level checkout.c as an illustration of the libification discussed in our recent threads.

Note that I consider this a fairly early and rough draft for illustration only. There may be a few topics that touch 'git checkout', and a refactoring of this scale might adversely impact them, so I will not even merge these patches to 'seen'.

 1/8: checkout: pass cb_option explicitly to branch name parsers
 2/8: checkout: validate new branch name in checkout_branch()
 3/8: checkout: validate stage and merge option compatibility in
        checkout_paths()
 4/8: checkout: extract option validation and pathspec helpers
 5/8: checkout: extract branch setup and tracking helpers
 6/8: checkout: restructure switch, restore, and checkout
        entrypoints
 7/8: checkout: wrap overly long lines
 8/8: checkout: move post_checkout_hook() to checkout.c

 builtin/checkout.c | 545 +++++++++++++++++++++++----------------------
 checkout.c         |  22 ++
 checkout.h         |   8 +
 3 files changed, 311 insertions(+), 264 deletions(-)