If the subrepo is tracking a branch and you reclone using

    git subrepo clone --force $URL

without specifying -b/--branch then the branch setting in .gitrepo is
not updated.

…HEAD

Before, there would be a cryptic error about invalid parameters,
because prev_commit would never get set.

This situation can occur when the commit that last touched the subrepo
has been rebased.

I did not find it immediately obvious how to gather the
information needed to fix a missing parent error. Search the
repository for the values needed and present them as part of the
message.

Fixes ingydotnet#617, ingydotnet#415, ingydotnet#600

Did not query git to determine the default branch name, assumed
it was master.

Fixes ingydotnet#587

Sourced-only scripts should not start with hashbangs.

Bash scripts that are not to be executed standalone should not have
executable permissions.

Using the 'make install' method, the default 'git-core' location
of git-subrepo executable does not automatically integrate git-subrepo
into git's own bash-completion. This change moves git-subrepo executable
with support scripts into /usr/share/git-subrepo. Then a symlink to the
'git-subrepo' executable script has been added into /usr/bin to achieve
recognition of the 'git subrepo' sub-command under the git bash-completion
(through git's: --list-cmds=...,other,...).

Additional adjustment of Makefile and git-subrepo script were made to
make Makefile more generic and to make DESTDIR usage together with
potentially overriden install vars more manageable.
By overriding INSTALL_LIB and INSTALL_EXT to the same path, we achieve
removal of the unnecessary 'git-subrepo.d' subdirectory inside the
/usr/share/git-subrepo path.

Note that non 'make install' ways of installation should work as
before but without the need for the GIT_SUBREPO_ROOT variable.
Potentially, if 'realpath' addition in git-sibrepo change isn't available
on all target systems, readlink could still be used instead.

- Remove all subrepo refs with clean --force
- Allow the error() function to accept multiple arguments
- Bug fixed where clone --force could change the tracked branch
- Give a detailed error message if the parent SHA could not be found
- Use the git configuration to determine the default branch name for the init command
- Add --force to fetch command
- Fix executable settings on installed files
- Remove shebangs from library only files
- Fix bash-completion for instances that used make install