strbuf: introduce strbuf_getline_{lf,nul}()

The strbuf_getline() interface allows a byte other than LF or NUL as
the line terminator, but this is only because I wrote these
codepaths anticipating that there might be a value other than NUL
and LF that could be useful when I introduced line_termination long
time ago.  No useful caller that uses other value has emerged.

By now, it is clear that the interface is overly broad without a
good reason.  Many codepaths have hardcoded preference to read
either LF terminated or NUL terminated records from their input, and
then call strbuf_getline() with LF or NUL as the third parameter.

This step introduces two thin wrappers around strbuf_getline(),
namely, strbuf_getline_lf() and strbuf_getline_nul(), and
mechanically rewrites these call sites to call either one of
them.  The changes contained in this patch are:

 * introduction of these two functions in strbuf.[ch]

 * mechanical conversion of all callers to strbuf_getline() with
   either '\n' or '\0' as the third parameter to instead call the
   respective thin wrapper.

After this step, output from "git grep 'strbuf_getline('" would
become a lot smaller.  An interim goal of this series is to make
this an empty set, so that we can have strbuf_getline_crlf() take
over the shorter name strbuf_getline().

Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -388,13 +388,25 @@ extern int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint);
  */
 extern int strbuf_getline(struct strbuf *, FILE *, int);
 
+/**
+ * The strbuf_getline*() family of functions share this signature, but
+ * have different line termination conventions.
+ */
+typedef int (*strbuf_getline_fn)(struct strbuf *, FILE *);
+
+/* Uses LF as the line terminator */
+extern int strbuf_getline_lf(struct strbuf *sb, FILE *fp);
+
+/* Uses NUL as the line terminator */
+extern int strbuf_getline_nul(struct strbuf *sb, FILE *fp);
+
 /*
- * Similar to strbuf_getline(), but uses '\n' as the terminator,
- * and additionally treats a '\r' that comes immediately before '\n'
- * as part of the terminator.
+ * Similar to strbuf_getline_lf(), but additionally treats a CR that
+ * comes immediately before the LF as part of the terminator.
  */
 extern int strbuf_getline_crlf(struct strbuf *, FILE *);
 
+
 /**
  * Like `strbuf_getline`, but keeps the trailing terminator (if
  * any) in the buffer.