strbuf: provide function to append whole lines

While the strbuf interface already provides functions to read a line
into it that completely replaces its current contents, we do not have an
interface that allows for appending lines without discarding current
contents.

Add a new function `strbuf_appendwholeline` that reads a line including
its terminating character into a strbuf non-destructively. This is a
preparatory step for git-update-ref(1) reading standard input line-wise
instead of as a block.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -502,6 +502,12 @@ int strbuf_getline(struct strbuf *sb, FILE *file);
  */
 int strbuf_getwholeline(struct strbuf *sb, FILE *file, int term);
 
+/**
+ * Like `strbuf_getwholeline`, but appends the line instead of
+ * resetting the buffer first.
+ */
+int strbuf_appendwholeline(struct strbuf *sb, FILE *file, int term);
+
 /**
  * Like `strbuf_getwholeline`, but operates on a file descriptor.
  * It reads one character at a time, so it is very slow.  Do not