Add new "--prefix=..." option

This allows the user to specify a prefix to be prepended to all refs in
an instruction sheet when building or inspecting an integration branch.

The intended use case is for secondary users using someone else's
instruction sheet, who may have the branches under a different ref
namespace; for example, the refs that are under refs/heads/ for a
maintainer may be under refs/remotes/origin/ for users.

This option can also be configured using the "integration.prefix"
configuration key.

The Bash completion script suggests the first component of each ref
under the refs/remotes/ hierarchy.

Signed-off-by: John Keeping <john@keeping.me.uk>
Improve reflog action descriptions

git-integration currently relies on setup_reflog_action prefixing all
changes with "integration: ", but this does not show what is happening
when refs are updated in all cases.  Improve this by passing an explicit
"-m" argument to "git update-ref" and "git symbolic-ref" and overriding
$GIT_REFLOG_ACTION when calling "git checkout" or "git reset" in order
to provide a better description of what's going on.

Signed-off-by: John Keeping <john@keeping.me.uk>
Merge branch 'reflog-improvments'

Improve logging of actions in the reflog.

* reflog-improvments:
  Improve reflog action descriptions
Merge branch 'prefix-option'

Add a way to apply a prefix (e.g. "origin/") to all refs in the
instruction sheet.

* prefix-option:
  Add new "--prefix=..." option