multiple calls of mbed update fail for branches containing a "/" character

We are using a local fork of mbed-os for our projects as we rely on an older version but have to backport some changes from time to time.

If we reference a certain commit in `mbed-os.lib` which also happens to be the HEAD of a certain branch, if the branch contains a `/` char in its name, `mbed update` fails if mbed-os has been checked out once before (see log at the end).

This is caused by mbed-cli trying to map revisions to refs and from there to branches.

If you issue a command (e.g., `mbed update; mbed deploy`, which triggers the checkout of a certain revision that also is the HEAD of a branch, this mapping normally results in one reference `refs/remotes/origin/`. This reference is correctly mapped to remote branch `origin/`.

What happens during the `checkout`, though, is that git also creates a reference with the identifier `refs/heads/`.

If you issue another command which triggers this procedure again, the code in [2] now finds two references `refs/remotes/origin/` and `refs/heads/`, cuts them to `origin/`and `´ (see [3]) and returns them with the latter one being the first in the returned list.

If now the branch contained a `/`, regex [3] will match for it and only the part after the slash will falsely be considered the branch name.
The comment for the match in [1] says: `# matches origin/ and isn't HEAD ref`
however, this time - as `origin` is omitted - this is not true, so a none-existing branch will be checkout and this obviously fails.

Fix: Regex in [3] should check for `origin/` instead of `/`. Then [4] can correctly identify and filter it.

[1] https://github.com/ARMmbed/mbed-cli/blob/76c92d07f77c8571e734e4ea122d51e93489893b/mbed/mbed.py#L783
[2} https://github.com/ARMmbed/mbed-cli/blob/76c92d07f77c8571e734e4ea122d51e93489893b/mbed/mbed.py#L910
[3] https://github.com/ARMmbed/mbed-cli/blob/76c92d07f77c8571e734e4ea122d51e93489893b/mbed/mbed.py#L782
[4] https://github.com/ARMmbed/mbed-cli/blob/76c92d07f77c8571e734e4ea122d51e93489893b/mbed/mbed.py#L784

Error log:
```
mbed update -vvvvv
[mbed] Working path "/home/xxx/git/xxx" (program)
[mbed] Updating program "xxx" to latest revision in the current branch
[mbed] Fetching revisions from remote repository to "xxx"
[mbed] Exec "git fetch --all --tags --force" in "/home/xxx/git/xxx"
Fetching origin
[mbed] Merging "xxx" with "origin/hotfix/set-lsedrv"
[mbed] Exec "git merge origin/hotfix/set-lsedrv" in "/home/xxx/git/LoRaTLSv2"
Already up to date.
[mbed] Updating library "mbed-os" to rev #e75915e866fb
[mbed] Fetching revisions from remote repository to "mbed-os"
[mbed] Exec "git fetch --all --tags --force" in "/home/xxx/git/xxx/mbed-os"
Fetching origin
[mbed] Checkout "e75915e866fbe17151a5e50048d4794805d42d04" in mbed-os
[mbed] Revision "e75915e866fbe17151a5e50048d4794805d42d04" matches a branch "set-lsedrv" reference. Re-associating with branch
[mbed] Exec "git checkout set-lsedrv -q" in "/home/xxx/git/xxx/mbed-os"
error: pathspec 'set-lsedrv' did not match any file(s) known to git
[mbed] ERROR: Unable to update "mbed-os" to rev #e75915e866fb

```

Start in mbed/mbed.py around lines 782-784 and 910, then reproduce the reported sequence with a branch whose name contains '/'. Verify how the revision-to-reference mapping behaves after a prior checkout; done means repeated mbed update or mbed deploy can reassociate the revision with the correct remote branch without the pathspec error.

Written by the indexing model from the issue text.