Fix version processing on BSD systems.

BSD systems ship with the original sed; lacking a host of GNU
extensions. However both GNU sed and POSIX sed support the -E flag
for extended regular expressions which are close to the change this
is replacing.

See the descriptions of extended REs in IEEE Std 1003.2 (aka POSIX.2)
or re_format(7) on BSD systems. What matters for this is that the
+ operator works and that it and the grouping operators no longer
need to be escaped.

completion: Complete only external command for 'run'.

Fix: list-tracked in subdir would print invalid paths

The difference is easiest to see when the following style is in effect:
.
	zstyle ':completion:*' group-name ''

Fix commit.

Fix parsing of Git version numbers

completion: Return the correct error code from __vcsh_*.

completion: Set the context correctly in 'foreach'.

completion: Pass GIT_DIR to _git.

Do not use shared Git repositories

docs: Clarify what 'enter' does.

Add bash-completion

Fix commit.

docs: Clarify what 'enter' does.

Add bash-completion

Fix parsing of Git version numbers

Fix: list-tracked in subdir would print invalid paths

Fixes #230

Shared repositories were created using `git init --shared=0600` with the intent
of keeping configuration data private. Git reports an error if "shared"
repositories are created with private permissions. (Due to an apparent bug, the
error was not reported before git-2.13.2.)

Instead of creating a shared repository, use `umask 0077` to make created files
accessible only to the current user. The umask setting is inherited by child
processes and respected by Git.