Support for git aliasing for tcsh completion

Support for git aliasing for tcsh completion

tcsh users sometimes alias the 'git' command to another name.  In
this case, the user expects to only have to issue a new 'complete'
command using the alias name.

However, the tcsh script currently uses the command typed by the
user to call the appropriate function in git-completion.bash, either
_git() or _gitk().  When using an alias, this technique no longer
works.

This change specifies the real name of the command (either 'git' or
'gitk') as a parameter to the script handling tcsh completion.  This
allows the user to use any alias for the 'git' or 'gitk' commands,
while still getting completion to work.

A check for the presence of ${HOME}/.git-completion.bash is also
added to help the user make use of the script properly.

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