checkout: add simple check for 'git checkout -b'

The 'git switch' command was created to separate half of the
behavior of 'git checkout'. It specifically has the mode to
do nothing with the index and working directory if the user
only specifies to create a new branch and change HEAD to that
branch. This is also the behavior most users expect from
'git checkout -b', but for historical reasons it also performs
an index update by scanning the working directory. This can be
slow for even moderately-sized repos.

A performance fix for 'git checkout -b' was introduced by
fa655d8411 (checkout: optimize "git checkout -b <new_branch>"
2018-08-16). That change includes details about the config
setting checkout.optimizeNewBranch when the sparse-checkout
feature is required. The way this change detected if this
behavior change is safe was through the skip_merge_working_tree()
method. This method was complex and needed to be updated
as new options were introduced.

This behavior was essentially reverted by 65f099b ("switch:
no worktree status unless real branch switch happens"
2019-03-29). Instead, two members of the checkout_opts struct
were used to distinguish between 'git checkout' and 'git switch':

    * switch_branch_doing_nothing_is_ok
    * only_merge_on_switching_branches

These settings have opposite values depending on if we start
in cmd_checkout or cmd_switch.

The message for 64f099b includes "Users of big repos are
encouraged to move to switch." Making this change while
'git switch' is still experimental is too aggressive.

Create a happy medium between these two options by making
'git checkout -b <branch>' behave just like 'git switch',
but only if we read exactly those arguments. This must
be done in cmd_checkout to avoid the arguments being
consumed by the option parsing logic.

This differs from the previous change by fa644d8 in that
the config option checkout.optimizeNewBranch remains
deleted. This means that 'git checkout -b' will ignore
the index merge even if we have a sparse-checkout file.
While this is a behavior change for 'git checkout -b',
it matches the behavior of 'git switch -c'.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Acked-by: Elijah Newren <newren@gmail.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -1698,6 +1698,15 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	opts.checkout_index = -2;    /* default on */
 	opts.checkout_worktree = -2; /* default on */
 
+	if (argc == 3 && !strcmp(argv[1], "-b")) {
+		/*
+		 * User ran 'git checkout -b <branch>' and expects
+		 * the same behavior as 'git switch -c <branch>'.
+		 */
+		opts.switch_branch_doing_nothing_is_ok = 0;
+		opts.only_merge_on_switching_branches = 1;
+	}
+
 	options = parse_options_dup(checkout_options);
 	options = add_common_options(&opts, options);
 	options = add_common_switch_branch_options(&opts, options);