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>