mbed import treats URLs differently differently than git clone

I would like to import from a git repo on a computer.
```
mbed import --protocol git user@host:path/to/repo/from/home/dir
```
This fails, because it tries to clone from `ssh://user@host/path/to/repo/from/home/dir`
It then proceeds to try mercurial, and complains that mercurial isn't installed in spite of passing `--protocol`
If you do
```
git clone user@host:path/to/repo/from/home/dir
```
It effectively expands (no idea how it actually does it) to `ssh://user@host/home/user/path/to/repo/from/home/dir`
So the correct command is
```
mbed import user@host:/home/user/path/to/repo/from/home/dir
```

The moral of this story is I did `mbed import --protocol git user@host:path/to/repo/from/home/dir` and got the error that mercurial was not installed.

I'm not sure what a good solution to this is, because we can't assume `/home/user`, correct?

It was just fairly confusing.

Also how to get help on a subcommand is really unclear.
`mbed help` makes sense, so I tried `mbed help import`, `mbed import help`, `mbed --help import`, and then asssumed there was no help for subcommands.
I proceeded to start reading the source to figure out how the protocol was set, and I found what should be help for subcommands.
I started reading up on how the argparser library worked to try to suggest a fix, and saw that some tutorial used the invocation `command subcommand --help`.
Maybe mention that in the results of `mbed help`

Start by reproducing the documented mbed import commands and then inspect the import subcommand source where protocol selection and repository URLs are handled. Read the argument-parser setup for subcommand help and compare its behavior with the commands described in the issue. Done means the reported URL and --protocol behavior is resolved or clearly reported, and mbed help points users to the supported subcommand help syntax.

Written by the indexing model from the issue text.