Merge branch 'jc/prune-all'

We used the approxidate() parser for "--expire=<timestamp>" options
of various commands, but it is better to treat --expire=all and
--expire=now a bit more specially than using the current timestamp.
Update "git gc" and "git reflog" with a new parsing function for
expiry dates.

* jc/prune-all:
  prune: introduce OPT_EXPIRY_DATE() and use it
  api-parse-options.txt: document "no-" for non-boolean options
  git-gc.txt, git-reflog.txt: document new expiry options
  date.c: add parse_expiry_date()

Documentation/technical/api-parse-options.txt

@@ -41,6 +41,8 @@ The parse-options API allows:
 * Boolean long options can be 'negated' (or 'unset') by prepending
   `no-`, e.g. `--no-abbrev` instead of `--abbrev`. Conversely,
   options that begin with `no-` can be 'negated' by removing it.
+  Other long options can be unset (e.g., set string to NULL, set
+  integer to 0) by prepending `no-`.
 
 * Options and non-option arguments can clearly be separated using the `--`
   option, e.g. `-a -b --option -- --this-is-a-file` indicates that
@@ -174,6 +176,10 @@ There are some macros to easily define options:
 	Introduce an option with date argument, see `approxidate()`.
 	The timestamp is put into `int_var`.
 
+`OPT_EXPIRY_DATE(short, long, &int_var, description)`::
+	Introduce an option with expiry date argument, see `parse_expiry_date()`.
+	The timestamp is put into `int_var`.
+
 `OPT_CALLBACK(short, long, &var, arg_str, description, func_ptr)`::
 	Introduce an option with argument.
 	The argument will be fed into the function given by `func_ptr`