parse_date_format(): convert a format name to an enum date_mode

Factor out the code to parse --date=<format> parameter to revision
walkers into a separate function, parse_date_format().  This function
is passed a string and converts it to an enum date_format:

 - "relative"         => DATE_RELATIVE
 - "iso8601" or "iso" => DATE_ISO8601
 - "rfc2822"          => DATE_RFC2822
 - "short"            => DATE_SHORT
 - "local"            => DATE_LOCAL
 - "default"          => DATE_NORMAL

In the event that none of these strings is found, the function die()s.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -432,6 +432,7 @@ const char *show_date(unsigned long time, int timezone, enum date_mode mode);
 int parse_date(const char *date, char *buf, int bufsize);
 void datestamp(char *buf, int bufsize);
 unsigned long approxidate(const char *);
+enum date_mode parse_date_format(const char *format);
 
 extern const char *git_author_info(int);
 extern const char *git_committer_info(int);

date.c

@@ -584,6 +584,26 @@ int parse_date(const char *date, char *result, int maxlen)
 	return date_string(then, offset, result, maxlen);
 }
 
+enum date_mode parse_date_format(const char *format)
+{
+	if (!strcmp(format, "relative"))
+		return DATE_RELATIVE;
+	else if (!strcmp(format, "iso8601") ||
+		 !strcmp(format, "iso"))
+		return DATE_ISO8601;
+	else if (!strcmp(format, "rfc2822") ||
+		 !strcmp(format, "rfc"))
+		return DATE_RFC2822;
+	else if (!strcmp(format, "short"))
+		return DATE_SHORT;
+	else if (!strcmp(format, "local"))
+		return DATE_LOCAL;
+	else if (!strcmp(format, "default"))
+		return DATE_NORMAL;
+	else
+		die("unknown date format %s", format);
+}
+
 void datestamp(char *buf, int bufsize)
 {
 	time_t now;

revision.c

@@ -1134,22 +1134,7 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 				continue;
 			}
 			if (!strncmp(arg, "--date=", 7)) {
-				if (!strcmp(arg + 7, "relative"))
-					revs->date_mode = DATE_RELATIVE;
-				else if (!strcmp(arg + 7, "iso8601") ||
-					 !strcmp(arg + 7, "iso"))
-					revs->date_mode = DATE_ISO8601;
-				else if (!strcmp(arg + 7, "rfc2822") ||
-					 !strcmp(arg + 7, "rfc"))
-					revs->date_mode = DATE_RFC2822;
-				else if (!strcmp(arg + 7, "short"))
-					revs->date_mode = DATE_SHORT;
-				else if (!strcmp(arg + 7, "local"))
-					revs->date_mode = DATE_LOCAL;
-				else if (!strcmp(arg + 7, "default"))
-					revs->date_mode = DATE_NORMAL;
-				else
-					die("unknown date format %s", arg);
+				revs->date_mode = parse_date_format(arg + 7);
 				continue;
 			}
 			if (!strcmp(arg, "--log-size")) {