builtin/merge_recursive.c: Add an usage string and make use of it.
This improves the usage output by adding builtin_merge_recursive_usage string
that follows the same pattern used by the other builtin commands.
The previous output for git merger-recursive was:
usage: merge-recursive <base>... -- <head> <remote> ...
Now the output is:
usage: git merge-recursive <base>... -- <head> <remote> ...
Since cmd_merge_recursive is used to handle four different commands we need
the %s in the usage string, so the following example:
$ git merge-subtree -h
Will output:
usage: git merge-subtree <base>... -- <head> <remote> ...
Signed-off-by: Thiago Farina <tfransosi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>