sq_quote_buf_pretty: don't drop empty arguments

Empty arguments passed on the command line can be represented by
a '', however sq_quote_buf_pretty was incorrectly dropping these
arguments altogether. Fix this problem by ensuring that such
arguments are emitted as '' instead.

Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

quote.c

@@ -48,6 +48,12 @@ void sq_quote_buf_pretty(struct strbuf *dst, const char *src)
 	static const char ok_punct[] = "+,-./:=@_^";
 	const char *p;
 
+	/* Avoid losing a zero-length string by adding '' */
+	if (!*src) {
+		strbuf_addstr(dst, "''");
+		return;
+	}
+
 	for (p = src; *p; p++) {
 		if (!isalpha(*p) && !isdigit(*p) && !strchr(ok_punct, *p)) {
 			sq_quote_buf(dst, src);

t/t0014-alias.sh

@@ -37,4 +37,11 @@ test_expect_success 'looping aliases - internal execution' '
 #	test_i18ngrep "^fatal: alias loop detected: expansion of" output
 #'
 
+test_expect_success 'run-command formats empty args properly' '
+    GIT_TRACE=1 git frotz a "" b " " c 2>&1 |
+    sed -ne "/run_command:/s/.*trace: run_command: //p" >actual &&
+    echo "git-frotz a '\'''\'' b '\'' '\'' c" >expect &&
+    test_cmp expect actual
+'
+
 test_done