Introduce git__date_rfc2822_fmt. Allows for RFC2822 date headers

src/date.c

@@ -874,3 +874,31 @@ int git__date_parse(git_time_t *out, const char *date)
 	*out = approxidate_str(date, time_sec, &error_ret);
    return error_ret;
 }
+
+int git__date_rfc2822_fmt(char *out, size_t len, const git_time *date)
+{
+	int written;
+	struct tm gmt;
+	time_t t;
+
+	assert(out && date);
+
+	t = (time_t) (date->time + date->offset * 60);
+
+	if (p_gmtime_r (&t, &gmt) == NULL)
+		return -1;
+
+	written = p_snprintf(out, len, "%.3s, %u %.3s %.4u %02u:%02u:%02u %+03d%02d",
+		weekday_names[gmt.tm_wday],
+		gmt.tm_mday,
+		month_names[gmt.tm_mon],
+		gmt.tm_year + 1900,
+		gmt.tm_hour, gmt.tm_min, gmt.tm_sec,
+		date->offset / 60, date->offset % 60);
+
+	if (written < 0 || (written > (int) len - 1))
+		return -1;
+
+	return 0;
+}
+

src/util.h

@@ -20,6 +20,8 @@
 # define max(a,b) ((a) > (b) ? (a) : (b))
 #endif
 
+#define GIT_DATE_RFC2822_SZ  32
+
 /*
  * Custom memory allocation wrappers
  * that set error code and error message
@@ -329,6 +331,16 @@ extern int git__parse_bool(int *out, const char *value);
 extern int git__date_parse(git_time_t *out, const char *date);
 
 /*
+ * Format a git_time as a RFC2822 string
+ *
+ * @param out buffer to store formatted date; a '\\0' terminator will automatically be added.
+ * @param len size of the buffer; should be atleast `GIT_DATE_RFC2822_SZ` in size;
+ * @param date the date to be formatted
+ * @return 0 if successful; -1 on error
+ */
+extern int git__date_rfc2822_fmt(char *out, size_t len, const git_time *date);
+
+/*
  * Unescapes a string in-place.
  *
  * Edge cases behavior: