pretty: Introduce ' ' modifier to add space if non-empty

We have the '+' modifiier which helps combine format specifiers which
may possibly be empty, e.g. '%s%+b%n'.

Introduce an analogous ' ' (space) modifier which adds a space before
non-empty items. This helps assemble "one line type" format specifiers.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -159,6 +159,10 @@ If you add a `-` (minus sign) after '%' of a placeholder, line-feeds that
 immediately precede the expansion are deleted if and only if the
 placeholder expands to an empty string.
 
+If you add a ` ` (space) after '%' of a placeholder, a space
+is inserted immediately before the expansion if and only if the
+placeholder expands to a non-empty string.
+
 * 'tformat:'
 +
 The 'tformat:' format works exactly like 'format:', except that it

pretty.c

@@ -942,6 +942,7 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 		NO_MAGIC,
 		ADD_LF_BEFORE_NON_EMPTY,
 		DEL_LF_BEFORE_EMPTY,
+		ADD_SP_BEFORE_NON_EMPTY,
 	} magic = NO_MAGIC;
 
 	switch (placeholder[0]) {
@@ -951,6 +952,9 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 	case '+':
 		magic = ADD_LF_BEFORE_NON_EMPTY;
 		break;
+	case ' ':
+		magic = ADD_SP_BEFORE_NON_EMPTY;
+		break;
 	default:
 		break;
 	}
@@ -965,8 +969,11 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 	if ((orig_len == sb->len) && magic == DEL_LF_BEFORE_EMPTY) {
 		while (sb->len && sb->buf[sb->len - 1] == '\n')
 			strbuf_setlen(sb, sb->len - 1);
-	} else if ((orig_len != sb->len) && magic == ADD_LF_BEFORE_NON_EMPTY) {
-		strbuf_insert(sb, orig_len, "\n", 1);
+	} else if (orig_len != sb->len) {
+		if (magic == ADD_LF_BEFORE_NON_EMPTY)
+			strbuf_insert(sb, orig_len, "\n", 1);
+		else if (magic == ADD_SP_BEFORE_NON_EMPTY)
+			strbuf_insert(sb, orig_len, " ", 1);
 	}
 	return consumed + 1;
 }
@@ -976,7 +983,7 @@ static size_t userformat_want_item(struct strbuf *sb, const char *placeholder,
 {
 	struct userformat_want *w = context;
 
-	if (*placeholder == '+' || *placeholder == '-')
+	if (*placeholder == '+' || *placeholder == '-' || *placeholder == ' ')
 		placeholder++;
 
 	switch (*placeholder) {

t/t6006-rev-list-format.sh

@@ -200,6 +200,16 @@ test_expect_success 'add LF before non-empty (2)' '
 	grep "^$" actual
 '
 
+test_expect_success 'add SP before non-empty (1)' '
+	git show -s --pretty=format:"%s% bThanks" HEAD^^ >actual &&
+	test $(wc -w <actual) = 2
+'
+
+test_expect_success 'add SP before non-empty (2)' '
+	git show -s --pretty=format:"%s% sThanks" HEAD^^ >actual &&
+	test $(wc -w <actual) = 4
+'
+
 test_expect_success '--abbrev' '
 	echo SHORT SHORT SHORT >expect2 &&
 	echo LONG LONG LONG >expect3 &&