parse-options: make CMDMODE errors more precise

parse-options: make CMDMODE errors more precise

Only a single PARSE_OPT_CMDMODE option can be specified for the same
variable at the same time.  This is enforced by get_value(), but the
error messages are imprecise in three ways:

1. If a non-PARSE_OPT_CMDMODE option changes the value variable of a
PARSE_OPT_CMDMODE option then an ominously vague message is shown:

   $ t/helper/test-tool parse-options --set23 --mode1
   error: option `mode1' : incompatible with something else

Worse: If the order of options is reversed then no error is reported at
all:

   $ t/helper/test-tool parse-options --mode1 --set23
   boolean: 0
   integer: 23
   magnitude: 0
   timestamp: 0
   string: (not set)
   abbrev: 7
   verbose: -1
   quiet: 0
   dry run: no
   file: (not set)

Fortunately this can currently only happen in the test helper; actual
Git commands don't share the same variable for the value of options with
and without the flag PARSE_OPT_CMDMODE.

2. If there are multiple options with the same value (synonyms), then
the one that is defined first is shown rather than the one actually
given on the command line, which is confusing:

   $ git am --resolved --quit
   error: option `quit' is incompatible with --continue

3. Arguments of PARSE_OPT_CMDMODE options are not handled by the
parse-option machinery.  This is left to the callback function.  We
currently only have a single affected option, --show-current-patch of
git am.  Errors for it can show an argument that was not actually given
on the command line:

   $ git am --show-current-patch --show-current-patch=diff
   error: options '--show-current-patch=diff' and '--show-current-patch=raw' cannot be used together

The options --show-current-patch and --show-current-patch=raw are
synonyms, but the error accuses the user of input they did not actually
made.  Or it can awkwardly print a NULL pointer:

   $ git am --show-current-patch=diff --show-current-patch
   error: options '--show-current-patch=(null)' and '--show-current-patch=diff' cannot be used together

The reasons for these shortcomings is that the current code checks
incompatibility only when encountering a PARSE_OPT_CMDMODE option at the
command line, and that it searches the previous incompatible option by
value.

Fix the first two points by checking all PARSE_OPT_CMDMODE variables
after parsing each option and by storing all relevant details if their
value changed.  Do that whether or not the changing options has the flag
PARSE_OPT_CMDMODE set.  Report an incompatibility only if two options
change the variable to different values and at least one of them is a
PARSE_OPT_CMDMODE option.  This changes the output of the first three
examples above to:

   $ t/helper/test-tool parse-options --set23 --mode1
   error: --mode1 is incompatible with --set23
   $ t/helper/test-tool parse-options --mode1 --set23
   error: --set23 is incompatible with --mode1
   $ git am --resolved --quit
   error: --quit is incompatible with --resolved

Store the argument of PARSE_OPT_CMDMODE options of type OPTION_CALLBACK
as well to allow taking over the responsibility for compatibility
checking from the callback function.  The next patch will use this
capability to fix the messages for git am --show-current-patch.

Use a linked list for storing the PARSE_OPT_CMDMODE variables.  This
somewhat outdated data structure is simple and suffices, as the number
of elements per command is currently only zero or one.  We do support
multiple different command modes variables per command, but I don't
expect that we'd ever use a significant number of them.  Once we do we
can switch to a hashmap.

Since we no longer need to search the conflicting option, the all_opts
parameter of get_value() is no longer used.  Remove it.

Extend the tests to check for both conflicting option names, but don't
insist on a particular order.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>