date: use strbufs in date-formatting functions

Many of the date functions write into fixed-size buffers.
This is a minor pain, as we have to take special
precautions, and frequently end up copying the result into a
strbuf or heap-allocated buffer anyway (for which we
sometimes use strcpy!).

Let's instead teach parse_date, datestamp, etc to write to a
strbuf. The obvious downside is that we might need to
perform a heap allocation where we otherwise would not need
to. However, it turns out that the only two new allocations
required are:

  1. In test-date.c, where we don't care about efficiency.

  2. In determine_author_info, which is not performance
     critical (and where the use of a strbuf will help later
     refactoring).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1044,10 +1044,10 @@ enum date_mode {
 const char *show_date(unsigned long time, int timezone, enum date_mode mode);
 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(const char *date, struct strbuf *out);
 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);
+void datestamp(struct strbuf *out);
 #define approxidate(s) approxidate_careful((s), NULL)
 unsigned long approxidate_careful(const char *, int *);
 unsigned long approxidate_relative(const char *date, const struct timeval *now);