This removes the requirement on `restic self-update --output` to point
to a path of an existing file, to overwrite. In case the specified
path does exist we still want to verify that it's a regular file,
rather than a directory or a device, which gets overwritten.
We also want to verify that a path to a new file exists within an
existing directory. The alternative being running into that issue
after the actual download, etc has completed.
While at it I also replace `errors.Errorf` with the more appropriately
verbose `errors.Fatalf`.
Resolves #2491
This commit adds a command called `self-update` which downloads the
latest released version of restic from GitHub and replacing the current
binary with it. It does not rely on any external program (so it'll work
everywhere), but still verifies the GPG signature using the embedded GPG
public key.
By default, the `self-update` command is hidden behind the `selfupdate`
built tag, which is only set when restic is built using `build.go`. The
reason for this is that downstream distributions will then not include
the command by default, so users are encouraged to use the
platform-specific distribution mechanism.
