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()

cache.h

@@ -910,6 +910,7 @@ void show_date_relative(unsigned long time, int tz, const struct timeval *now,
 			struct strbuf *timebuf);
 int parse_date(const char *date, char *buf, int bufsize);
 int parse_date_basic(const char *date, unsigned long *timestamp, int *offset);
+int parse_expiry_date(const char *date, unsigned long *timestamp);
 void datestamp(char *buf, int bufsize);
 #define approxidate(s) approxidate_careful((s), NULL)
 unsigned long approxidate_careful(const char *, int *);