install: GitHub branches with forward slashes installs correctly but give wrong output

[HenrikBengtsson]

Installing GitHub branches with forward slashes installs correctly but give wrong output. For example:

> source("http://callr.org/install#HenrikBengtsson/R.matlab@hotfix/tempfiles")
Installing/updating package: 'tempfiles'
Installing packages from GitHub:
Downloading GitHub repo HenrikBengtsson/R.matlab@hotfix/tempfiles
Installing R.matlab
"C:/PROGRA~1/R/R-3.2.2patched/bin/x64/R" --no-site-file --no-environ  \
  --no-save --no-restore CMD INSTALL  \
  "C:/Users/hb/AppData/Local/Temp/RtmpCaxEym/devtools21085d2d1711/HenrikBengtsson-R.matlab-5d9e9dc"  \
  --library="C:/Users/hb/R/win-library/3.2" --install-tests

* installing *source* package 'R.matlab' ...
** R
** inst
** tests
** byte-compile and prepare package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded
*** arch - i386
*** arch - x64
* DONE (R.matlab)

Package updates:
            Previous Current
R.matlab  3.2.0-9000 3.2.0-1
tempfiles       <NA>    <NA>

Failed to install requested package: 'tempfiles'

Note how install get confused about what the actual package name is.

[HenrikBengtsson]

Fixed.