strbuf: introduce strbuf_addstrings() to repeatedly add a string

In a following commit we are going to port code from
"t/helper/test-sha256.c", t/helper/test-hash.c and "t/t0015-hash.sh" to
a new "t/unit-tests/t-hash.c" file using the recently added unit test
framework.

To port code like: perl -e "$| = 1; print q{aaaaaaaaaa} for 1..100000;"
we are going to need a new strbuf_addstrings() function that repeatedly
adds the same string a number of times to a buffer.

Such a strbuf_addstrings() function would already be useful in
"json-writer.c" and "builtin/submodule-helper.c" as both of these files
already have code that repeatedly adds the same string. So let's
introduce such a strbuf_addstrings() function in "strbuf.{c,h}" and use
it in both "json-writer.c" and "builtin/submodule-helper.c".

We use the "strbuf_addstrings" name as this way strbuf_addstr() and
strbuf_addstrings() would be similar for strings as strbuf_addch() and
strbuf_addchars() for characters.

Helped-by: Junio C Hamano <gitster@pobox.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Co-authored-by: Achu Luma <ach.lumap@gmail.com>
Signed-off-by: Achu Luma <ach.lumap@gmail.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -310,6 +310,11 @@ static inline void strbuf_addstr(struct strbuf *sb, const char *s)
 	strbuf_add(sb, s, strlen(s));
 }
 
+/**
+ * Add a NUL-terminated string the specified number of times to the buffer.
+ */
+void strbuf_addstrings(struct strbuf *sb, const char *s, size_t n);
+
 /**
  * Copy the contents of another buffer at the end of the current one.
  */