Show error when the user tries neo.py update in a detached HEAD state...

[screamerbg]

...and encourage to manually switch to a branch (gits behavior in detached state)

Fixed issue #58 and also allow keeping of orphaned references (.lib without clone repo) during update to allow import behavior

[geky]

+1 from me

[screamerbg]

@bogdanm ?

[0xc0170]

I tested this for #56 and #58 issues, both fixed with this patch.

[0xc0170]

One regression I found, use-case update running on not yet deployed mbed-os:

Without this patch:

python C:\Code\git_repo\github\neo\neo.py update
[neo.py] Working path "C:\2" (program)
[neo.py] Synchronizing dependency references...
[neo.py] Remove reference "mbed-os" -> "https://github.com/ARMmbed/mbed-os.git/#7996649d966f"
[neo.py] Removing C:\2\mbed-os.lib
[neo.py] "git rm -f C:\2\mbed-os.lib"
fatal: pathspec 'C:\2\mbed-os.lib' did not match any files
[neo.py] Fetching remote repository "2" to local "2" and updating to latest revision in the current branch
[neo.py] "git pull -v --all"
You asked to pull from the remote '--all', but did not specify
a branch. Because this is not the default configured remote
for your current branch, you must specify a branch on the command line.
[neo.py ERROR] Subrocess exit with error code 1
---

with this patch, traceback:

python neo.py update
[neo.py] Working path "C:\2" (program)
[neo.py] Synchronizing dependency references...
Traceback (most recent call last):
  File "C:\Code\git_repo\github\neo\neo.py", line 1114, in <module>
    status = args.command(args)
  File "C:\Code\git_repo\github\neo\neo.py", line 688, in thunk
    return command(**argv)
  File "C:\Code\git_repo\github\neo\neo.py", line 862, in update
    sync(keep_refs=True)
  File "C:\Code\git_repo\github\neo\neo.py", line 964, in sync
    with cd(lib.path):
  File "C:\Python27\lib\contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "C:\Code\git_repo\github\neo\neo.py", line 164, in cd
    os.chdir(newdir)
WindowsError: [Error 2] The system cannot find the file specified: 'C:\\2\\mbed-os'

Should update run deploy if we know that the lib was not yet deployed, or just print an error to run deploy first prior update (not all libraries are deployed).

[screamerbg]

This should be now fixed. Thanks for the bug report