short help: allow a gap smaller than USAGE_GAP

The parse-options API responds to "git cmd -h" by listing the option
flag (padded to the USAGE_OPTS_WIDTH column), followed by USAGE_GAP
(set to 2) whitespaces, followed by the help text.  If the flags
part does not fit within the USAGE_OPTS_WIDTH, the help text is given
on its own line.  Imagine that "@" below depicts the USAGE_OPTS_WIDTH'th
column, and "#" are for the usage help text, the output may look
like this:

    @@@@@@@@@@@@@  ########################################
    -f		   description of the flag '-f' comes here
    --short=<num>  description of the flag '--short'
    --very-long-option=<number>
                   description of the flag '--very-long-option'

This is all good and nice in principle, but it becomes awkward when
the flags part is just one column over the limit and forces a line
break.  See the description of the "--almost" option below:

    @@@@@@@@@@@@@  ########################################
    -f		   description of the flag '-f' comes here
    --short=<num>  description of the flag '--short'
    --almost=<num>
                   description of the flag '--almost'
    --very-long-option=<number>
                   description of the flag '--very-long-option'

If we allow shrinking the gap to a single whitespace only in such a
case, we would instead get:

    @@@@@@@@@@@@@  ########################################
    -f		   description of the flag '-f' comes here
    --short=<num>  description of the flag '--short'
    --almost=<num> description of the flag '--almost'
    --very-long-option=<number>
                   description of the flag '--very-long-option'

and the boundary between the flags and their descriptions does not
become any harder to see, while saving precious vertical screen real
estate.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.c

@@ -1146,7 +1146,9 @@ static enum parse_opt_result usage_with_options_internal(struct parse_opt_ctx_t
 		    !(opts->flags & PARSE_OPT_NOARG))
 			pos += usage_argh(opts, outfile);
 
-		if (pos <= USAGE_OPTS_WIDTH)
+		if (pos == USAGE_OPTS_WIDTH + 1)
+			pad = -1;
+		else if (pos <= USAGE_OPTS_WIDTH)
 			pad = USAGE_OPTS_WIDTH - pos;
 		else {
 			fputc('\n', outfile);

t/t0040-parse-options.sh

@@ -30,8 +30,7 @@ usage: test-tool parse-options <options>
     -F, --file <file>     set file to <file>
 
 String options
-    -s, --string <string>
-                          get a string
+    -s, --string <string> get a string
     --string2 <str>       get another string
     --st <st>             get another string (pervert ordering)
     -o <str>              get another string