help -a: handle aliases with long names gracefully

We take pains to determine the longest command beforehand, so that we
can align the category column after printing the command names.

However, then we re-use that value when printing the aliases. If any
alias name is longer than the longest command name, we consequently try
to add a negative number of spaces (but `mput_char()` does not expect
any negative values and simply decrements until the value is 0, i.e.
it tries to add close to 2**31 spaces).

Let's fix this by adjusting the `longest` variable before printing the
aliases.

This fixes https://github.com/git-for-windows/git/issues/1975.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -83,8 +83,9 @@ static void print_command_list(const struct cmdname_help *cmds,
 
 	for (i = 0; cmds[i].name; i++) {
 		if (cmds[i].category & mask) {
+			size_t len = strlen(cmds[i].name);
 			printf("   %s   ", cmds[i].name);
-			mput_char(' ', longest - strlen(cmds[i].name));
+			mput_char(' ', longest > len ? longest - len : 1);
 			puts(_(cmds[i].help));
 		}
 	}
@@ -526,6 +527,13 @@ void list_all_cmds_help(void)
 
 	git_config(get_alias, &alias_list);
 	string_list_sort(&alias_list);
+
+	for (i = 0; i < alias_list.nr; i++) {
+		size_t len = strlen(alias_list.items[i].string);
+		if (longest < len)
+			longest = len;
+	}
+
 	if (alias_list.nr) {
 		printf("\n%s\n", _("Command aliases"));
 		ALLOC_ARRAY(aliases, alias_list.nr + 1);