ident: let callers omit name with fmt_indent

Most callers want to see all of "$name <$email> $date", but
a few want only limited parts, omitting the date, or even
the name. We already have IDENT_NO_DATE to handle the date
part, but there's not a good option for getting just the
email. Callers have to done one of:

  1. Call ident_default_email; this does not respect
     environment variables, nor does it promise to trim
     whitespace or other crud from the result.

  2. Call git_{committer,author}_info; this returns the name
     and email, leaving the caller to parse out the wanted
     bits.

This patch adds IDENT_NO_NAME; it stops short of adding
IDENT_NO_EMAIL, as no callers want it (nor are likely to),
and it complicates the error handling of the function.

When no name is requested, the angle brackets (<>) around
the email address are also omitted.

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

cache.h

@@ -889,6 +889,7 @@ enum date_mode parse_date_format(const char *format);
 
 #define IDENT_ERROR_ON_NO_NAME 1
 #define IDENT_NO_DATE	       2
+#define IDENT_NO_NAME	       4
 extern const char *git_author_info(int);
 extern const char *git_committer_info(int);
 extern const char *fmt_ident(const char *name, const char *email, const char *date_str, int);