No Multiple Selection Available When Adding Local Repositories

### Description

It appears one cannot select multiple local repositories to add to GitHub Desktop from the relevant file/folder picker at the same time as one could in an earlier version of the app.

### Version

**GitHub Desktop version:** v1.0.3

**OS version:** OS X v10.11.6 'El Capitan'

### Steps to Reproduce

1. Either:
1. Select 'Add Local Repository…' from the 'File' menu, or…
1. …invoke the keyboard shortcut assigned to it by pressing '⌘O.'
1. Click the resulting modal dialog sheet's 'Choose…' button.
1. Attempt to select multiple GitHub repositories from a location within a mounted filesystem, such as your root one/boot drive, by Command- and/or Shift-clicking.

**Expected behavior:** The sequence of steps described above would select multiple local Git repositories for addition to GitHub Desktop.

**Actual behavior:** Selection of multiple repositories beyond the one initially clicked did not result in the addition of any more repositories to the list of selected ones.

**Reproduces how often:** Always, as far as I can tell from what I've seen so far.

### Logs

/Library/Application Support/GitHub Desktop/logs/2017-10-06.desktop.production.log

```
2017-10-06T19:26:48.363Z - info: [ui] launching: 1.0.3 (Mac OS 10.11.6)
2017-10-06T19:29:49.647Z - info: [ui] Executing isUsingLFS: git lfs track (took 1.701s)
2017-10-06T19:29:50.478Z - info: [ui] Executing getStatus: git status --untracked-files=all --branch --porcelain=2 -z (took 2.430s)
2017-10-06T19:29:50.568Z - info: [ui] Executing getStatus: git status --untracked-files=all --branch --porcelain=2 -z (took 2.127s)
2017-10-06T19:29:50.712Z - info: [ui] Executing getStatus: git status --untracked-files=all --branch --porcelain=2 -z (took 1.061s)
2017-10-06T19:29:53.156Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 1.823s)
2017-10-06T19:30:34.163Z - info: [ui] Executing getStatus: git status --untracked-files=all --branch --porcelain=2 -z (took 1.966s)
2017-10-06T19:30:34.209Z - info: [ui] Executing getStatus: git status --untracked-files=all --branch --porcelain=2 -z (took 1.971s)
2017-10-06T19:30:35.373Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 1.074s)
2017-10-06T19:31:31.929Z - info: [ui] Executing isUsingLFS: git lfs track (took 1.289s)
2017-10-06T19:31:33.845Z - info: [ui] Executing getRecentBranches: git log -g --no-abbrev-commit --pretty=oneline HEAD -n 2500 -- (took 1.893s)
2017-10-06T19:31:35.400Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 1.077s)
2017-10-06T19:31:35.527Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 1.596s)
2017-10-06T19:31:50.026Z - info: [ui] Executing getRecentBranches: git log -g --no-abbrev-commit --pretty=oneline HEAD -n 2500 -- (took 3.145s)
2017-10-06T19:31:50.069Z - info: [ui] Executing getRecentBranches: git log -g --no-abbrev-commit --pretty=oneline HEAD -n 2500 -- (took 3.105s)
2017-10-06T19:31:58.627Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune dshadowwolf (took 8.400s)
2017-10-06T19:32:09.429Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 5.212s)
2017-10-06T19:32:28.115Z - info: [ui] Executing getRecentBranches: git log -g --no-abbrev-commit --pretty=oneline HEAD -n 2500 -- (took 2.532s)
2017-10-06T19:32:28.116Z - info: [ui] Executing getRecentBranches: git log -g --no-abbrev-commit --pretty=oneline HEAD -n 2500 -- (took 2.409s)
2017-10-06T19:32:29.624Z - info: [ui] Executing getBranches: git for-each-ref --format=%(refname)%00%(refname:short)%00%(upstream:short)%00%(objectname)%00%(author)%00%(parent)%00%(subject)%00%(body)%00%1F refs/heads refs/remotes (took 3.937s)
2017-10-06T19:32:29.631Z - info: [ui] Executing getBranches: git for-each-ref --format=%(refname)%00%(refname:short)%00%(upstream:short)%00%(objectname)%00%(author)%00%(parent)%00%(subject)%00%(body)%00%1F refs/heads refs/remotes (took 4.065s)
2017-10-06T19:32:30.904Z - info: [ui] Executing getStatus: git status --untracked-files=all --branch --porcelain=2 -z (took 5.353s)
2017-10-06T19:32:30.936Z - info: [ui] Executing getStatus: git status --untracked-files=all --branch --porcelain=2 -z (took 5.266s)
2017-10-06T19:32:32.055Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 1.015s)
2017-10-06T19:32:55.958Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 1.462s)
2017-10-06T19:33:11.064Z - info: [ui] Executing getCommits: git log HEAD --date=raw --max-count=100 --pretty=%H%x1F%s%x1F%b%x1F%an <%ae> %ad%x1F%P -z --no-color (took 1.131s)
2017-10-06T19:33:12.456Z - info: [ui] Executing getRecentBranches: git log -g --no-abbrev-commit --pretty=oneline HEAD -n 2500 -- (took 3.965s)
2017-10-06T19:33:12.501Z - info: [ui] Executing getRecentBranches: git log -g --no-abbrev-commit --pretty=oneline HEAD -n 2500 -- (took 3.818s)
2017-10-06T19:33:13.588Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 1.021s)
2017-10-06T19:33:32.145Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune origin (took 1.208s)
```