Add a 'GetRemoteUrl()' function to 'git.GitHelper', which calls 'git remote
get-url origin' on a given repository path and returns the whitespace-
trimmed remote URL. Neither stdout nor stderr are printed to the console,
but the stderr text is included in the error message if Git exits with an
error status.

Although it is not used in this patch, 'GetRemoteUrl()' will be used in
later commits to display information about the configuration of the bundle
server.

Signed-off-by: Victoria Dye <vdye@github.com>

Add a new 'git-bundle-server' command to list the routes that are currently
configured and their remote Git URLs. Update manpage and 'README.md' with
description and usage information.

Due to how routes are currently stored in the bundle server, only "active"
routes are listed (that is, routes that have not been stopped with
'git-bundle-server stop'). This is intended to be only a temporary issue,
with future updates to how routes are stored allowing better tracking of
"stopped" routes.

Signed-off-by: Victoria Dye <vdye@github.com>

Add the 'ReadDirRecursive()' function to 'FileSystem' to facilitate easy
lookup of filesystem entries at a fixed depth from the root. In later
commits, this will be used to look up routes in the bundle server under the
'reporoot' on the filesystem.

'ReadDirRecursive()' takes three arguments:
* 'path': the root directory into which we want to recurse
* 'depth': the number of times to recurse into 'path'. A depth of 0 will
  return nothing, a depth of 1 will return all entries inside 'path', a
  depth of 2 will return all entries inside the subdirectories of 'path',
  etc.
* 'strictDepth': a flag indicating whether to return only entries that are
  *exactly 'depth' levels beneath 'path'. If 'false', 'ReadDirRecursive()'
  will also include files and empty directories at depths less than 'depth'.

For example, consider the following directory tree:

.
├── another-dir/
│   └── testfile
├── myfile
├── org/
│   └── repo/
│       ├── deeper/
│       └── repofile
└── too-shallow/

'ReadDirRecursive(".", 2, true)' will return:
- ./another-dir/testfile
- ./org/repo/

Whereas 'ReadDirRecursive(".", 2, false)' will return:
- ./another-dir/testfile
- ./myfile
- ./org/repo/
- ./too-shallow/

Signed-off-by: Victoria Dye <vdye@github.com>

Replace manual path joining via string concatenation with use of
'filepath.Join()'. The path normalization provided by the function helps
avoid issues caused by missing (or extra) path separators and aligns paths
to the to the appropriate filesystem's conventions.

Signed-off-by: Victoria Dye <vdye@github.com>

Add a function to the 'core.RepositoryProvider' that reads the on-disk
repository storage for the bundle server and returns the valid repos in a
route -> 'core.Repository' map.

'ReadRepositoryStorage()' identifies repositories by finding the directories
at a depth of 2 from the repo storage root, then checking that the
repository has a remote 'origin'. The result includes the "active" routes in
the bundle server (stored in the 'routes' file) as well as those that have
been removed from the routes file with 'git-bundle-server stop'. In later
commits, this information will be used to provide more complete information
about the state of the bundle server and configure "repairs" to a corrupted
server.

Signed-off-by: Victoria Dye <vdye@github.com>

Rename the 'core.RepositoryProvider' function 'writeRouteFile()' to
'WriteAllRoutes()' to make it accessible from other packages. The ability to
make "bulk" changes to the repository registry and commit all at once will
be useful in future patches, such as rebuilding the route list based on the
repositories on disk.

Signed-off-by: Victoria Dye <vdye@github.com>

Create a new command 'repair' to fix common issues with the bundle server's
internal storage. Start by implementing the 'routes' subcommand, which (by
default) removes routes from the 'routes' file if they do not appear (or are
not Git repositories) on disk.

Include two additional options:
- '--dry-run', which gathers repository information and constructs the list
  of repairs needed, but does not update anything
- '--start-all', which adds any unregistered, valid repositories on disk to
  the 'routes' file (effectively running 'git-bundle-server start' on all)

Signed-off-by: Victoria Dye <vdye@github.com>