Fix bug with multiple iconv conversions in one dir

The internal buffer in the `git_path_iconv_t` structure was not
being reset before the calls to `iconv` were made to convert data,
so if there were multiple decomposed Unicode paths in a single
directory, paths after the first one were being appended to the
first instead of treated as independent data.

src/path.c

@@ -782,6 +782,8 @@ int git_path_iconv(git_path_iconv_t *ic, char **in, size_t *inlen)
 		!git_path_has_non_ascii(*in, *inlen))
 		return 0;
 
+	git_buf_truncate(&ic->buf, 0);
+
 	while (1) {
 		if (git_buf_grow(&ic->buf, wantlen + 1) < 0)
 			return -1;

tests/core/iconv.c

@@ -39,8 +39,9 @@ void test_core_iconv__decomposed_to_precomposed(void)
 {
 #ifdef GIT_USE_ICONV
 	char *data = nfd;
-	size_t datalen = strlen(nfd);
+	size_t datalen, nfdlen = strlen(nfd);
 
+	datalen = nfdlen;
 	cl_git_pass(git_path_iconv(&ic, &data, &datalen));
 	GIT_UNUSED(datalen);
 
@@ -48,6 +49,15 @@ void test_core_iconv__decomposed_to_precomposed(void)
 	 * (on platforms where iconv is enabled, of course).
 	 */
 	cl_assert_equal_s(nfc, data);
+
+	/* should be able to do it multiple times with the same git_path_iconv_t */
+	data = nfd; datalen = nfdlen;
+	cl_git_pass(git_path_iconv(&ic, &data, &datalen));
+	cl_assert_equal_s(nfc, data);
+
+	data = nfd; datalen = nfdlen;
+	cl_git_pass(git_path_iconv(&ic, &data, &datalen));
+	cl_assert_equal_s(nfc, data);
 #endif
 }