tcsh-completion re-using git-completion.bash

tcsh-completion re-using git-completion.bash

The current tcsh-completion support for Git, as can be found on the
Internet, takes the approach of defining the possible completions
explicitly.  This has the obvious draw-back to require constant
updating as the Git code base evolves.

The approach taken by this commit is to to re-use the advanced bash
completion script and use its result for tcsh completion.  This is
achieved by sourcing the bash script and outputting the completion
result for tcsh consumption.

Three solutions were looked at to implement this approach with (C)
being retained:

  A) Modifications:
          git-completion.bash and new git-completion.tcsh

     Modify the existing git-completion.bash script to support
     being sourced using bash (as now), but also executed using bash.
     When being executed, the script will output the result of the
     computed completion to be re-used elsewhere (e.g., in tcsh).

     The modification to git-completion.bash is made not to be
     tcsh-specific, but to allow future users to also re-use its
     output.  Therefore, to be general, git-completion.bash accepts a
     second optional parameter, which is not used by tcsh, but could
     prove useful for other users.

     Pros:
       1- allows the git-completion.bash script to easily be re-used
       2- tcsh support is mostly isolated in git-completion.tcsh
     Cons (for tcsh users only):
       1- requires the user to copy both git-completion.tcsh and
          git-completion.bash to ${HOME}
       2- requires bash script to have a fixed name and location:
          ${HOME}/.git-completion.bash

  B) Modifications:
          git-completion.bash

     Modify the existing git-completion.bash script to support
     being sourced using bash (as now), but also executed using bash,
     and sourced using tcsh.

     Pros:
       1- only requires the user to deal with a single file
       2- maintenance more obvious for tcsh since it is entirely part
          of the same git-completion.bash script.
     Cons:
       1- tcsh support could affect bash support as they share the
          same script
       2- small tcsh section must use syntax suitable for both tcsh
          and bash and must be at the beginning of the script
       3- requires script to have a fixed name and location:
          ${HOME}/.git-completion.sh (for tcsh users only)

  C) Modifications:
          New git-completion.tcsh

     Provide a short tcsh script that generates another script
     which extends git-completion.bash.  This new script can be
     used by tcsh to perform completion.

     Pros:
       1- tcsh support is entirely isolated in git-completion.tcsh
       2- new tcsh script can be as complex as needed
     Cons (for tcsh users only):
       1- requires the user to copy both git-completion.tcsh and
          git-completion.bash to ${HOME}
       2- requires bash script to have a fixed name and location:
          ${HOME}/.git-completion.bash
       3- sourcing the new script will generate a third script

Approach (C) was selected avoid any modification to git-completion.bash.

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>
Reviewed-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>