generate-cmds.sh: export all commands to command-list.h

generate-cmds.sh: export all commands to command-list.h

The current generate-cmds.sh generates just enough to print "git help"
output. That is, it only extracts help text for common commands.

The script is now updated to extract help text for all commands and
keep command classification a new file, command-list.h. This will be
useful later:

- "git help -a" could print a short summary of all commands instead of
  just the common ones.

- "git" could produce a list of commands of one or more category. One
  of its use is to reduce another command classification embedded in
  git-completion.bash.

The new file can be generated but is not used anywhere yet. The plan
is we migrate away from common-cmds.h. Then we can kill off
common-cmds.h build rules and generation code (and also delete
duplicate content in command-list.h which we keep for now to not mess
generate-cmds.sh up too much).

PS. The new fixed column requirement on command-list.txt is
technically not needed. But it helps simplify the code a bit at this
stage. We could lift this restriction later if we want to.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>