Use proper syntax for replaceables in command docs

Use proper syntax for replaceables in command docs

The standard for command documentation synopses appears to be:

  [...] means optional
  <...> means replaceable
  [<...>] means both optional and replaceable

So fix a number of doc pages that use incorrect variations of the
above.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>