Refactor

This refactors a huge chunk of the code base to make it more maintainable.

Main points:

* Proper separation between bin and lib. Bin handles argument parsing &
  validation and (most of) the output. Lib provides interfaces for all
  opreations.

* Before, libgit2 internals were literred throughout the codebase,
  mainly the `Repository` struct and `git2::Error` in Results. They
  library is now properly wrapped in `repo.rs`, which exposes only the
  required functionality. It also standardizes the Error messages
  (they're just Strings for now) and handles stuff like the copious
  usage of Options to wrap maybe-invalid-utf-8 values. The program will
  still panic on non-utf-8 Strings e.g. in git remotes, but I guess this
  is acceptable. If you actually manage to hit this case, I promise I'll
  fix it :D

* Many unwraps() are now gone and properly handled.

* The table printing functionality is now confined to `table.rs`,
  instead of passing tables as parameters through the whole program.

tests/repo.rs

@@ -8,13 +8,13 @@ use helpers::*;
 fn open_empty_repo() {
     let tmpdir = init_tmpdir();
     assert!(matches!(
-        open_repo(tmpdir.path(), true),
+        RepoHandle::open(tmpdir.path(), true),
         Err(RepoError {
             kind: RepoErrorKind::NotFound
         })
     ));
     assert!(matches!(
-        open_repo(tmpdir.path(), false),
+        RepoHandle::open(tmpdir.path(), false),
         Err(RepoError {
             kind: RepoErrorKind::NotFound
         })
@@ -25,7 +25,7 @@ fn open_empty_repo() {
 #[test]
 fn create_repo() -> Result<(), Box<dyn std::error::Error>> {
     let tmpdir = init_tmpdir();
-    let repo = init_repo(tmpdir.path(), false)?;
+    let repo = RepoHandle::init(tmpdir.path(), false)?;
     assert!(!repo.is_bare());
     assert!(repo.is_empty()?);
     cleanup_tmpdir(tmpdir);
@@ -35,7 +35,7 @@ fn create_repo() -> Result<(), Box<dyn std::error::Error>> {
 #[test]
 fn create_repo_with_worktree() -> Result<(), Box<dyn std::error::Error>> {
     let tmpdir = init_tmpdir();
-    let repo = init_repo(tmpdir.path(), true)?;
+    let repo = RepoHandle::init(tmpdir.path(), true)?;
     assert!(repo.is_bare());
     assert!(repo.is_empty()?);
     cleanup_tmpdir(tmpdir);