Merge branch 'jk/date-mode-format'

Teach "git log" and friends a new "--date=format:..." option to
format timestamps using system's strftime(3).

* jk/date-mode-format:
  strbuf: make strbuf_addftime more robust
  introduce "format" date-mode
  convert "enum date_mode" into a struct
  show-branch: use DATE_RELATIVE instead of magic number

strbuf.h

@@ -344,6 +344,11 @@ extern void strbuf_commented_addf(struct strbuf *sb, const char *fmt, ...);
 __attribute__((format (printf,2,0)))
 extern void strbuf_vaddf(struct strbuf *sb, const char *fmt, va_list ap);
 
+/**
+ * Add the time specified by `tm`, as formatted by `strftime`.
+ */
+extern void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm);
+
 /**
  * Read a given size of data from a FILE* pointer to the buffer.
  *