feat: use current and default branches if none specified

If the user omits the source branch, use the current branch.
If the user omits the dest branch, use the default branch.
Resolves #3.

README.md

@@ -11,8 +11,13 @@ is the **destination** branch, which you're search through for the commits in.
 If a commit is present in source, but not in destination, that commit is
 considered "missing".
 
+If you don't specify the source, the current branch will be used, and if you don't
+specify the destination, the local copy of the default branch (e.g. `main`) will be
+used. Thus, `branch-detective` without any arguments will display the commits present
+in the current branch, but absent from `main`.
+
 To keep things simple, Branch Detective will not tell you about commits
-present in the desintation branch, but not in the source branch. To see these,
+present in the destination branch, but not in the source branch. To see these,
 rerun Branch Detective with the branches swapped.
 
 ## Search by Message or SHA

src/branch_detective/__main__.py

@@ -35,10 +35,12 @@ def verify_date(date: str, arg_name: str) -> Optional[datetime]:
 which commits are present on a SOURCE branch, but are absent from a DEST
 branch.""")
 @click.argument(
-    'source_branch'
+    'source_branch',
+    default=''
 )
 @click.argument(
-    'dest_branch'
+    'dest_branch',
+    default=''
 )
 @click.option(
     '--by-message/--by-sha', default=True,
@@ -83,6 +85,10 @@ def main(
 
     try:
         repo = RepositoryLens(source_branch, dest_branch)
+        # update branches to those detected by the repository
+        source_branch = repo.source_branch
+        dest_branch = repo.dest_branch
+        # get the commit logs
         source_log = repo.source_log
         dest_log = repo.dest_log
     except RuntimeError as e:

src/branch_detective/repository.py

@@ -8,7 +8,7 @@ class RepositoryLens:
     """Provides an interface to the repository in the current working
     directory."""
 
-    def __init__(self, source_branch: str, dest_branch: str):
+    def __init__(self, source_branch: str = '', dest_branch: str = ''):
         """Initializes a new RepositoryLens that works with the Git
         repository in the current working directory, and which specifically
         verifies and examines the source and destination branches specified
@@ -24,11 +24,23 @@ def __init__(self, source_branch: str, dest_branch: str):
         except git.exc.InvalidGitRepositoryError:
             raise RuntimeError("This is not a valid Git repository.")
 
+        # use current branch if source is not specified
+        if source_branch == '':
+            source_branch = self.repo.active_branch.name
+
         # Verify the requested 'source' branch is valid.
         self.source_branch: str = source_branch
         if source_branch not in self.repo.heads:
             raise RuntimeError(f"Unknown source branch: {source_branch}")
 
+        # use default branch if dest is not specified
+        if dest_branch == '':
+            try:
+                default = [r.ref for r in self.repo.refs if r.name == 'origin/HEAD'][0]
+                dest_branch = default.name.split('/')[-1]
+            except IndexError as e:
+                raise RuntimeError(f"Could not determine default (origin/HEAD) branch.")
+
         # Verify the requested 'dest' branch is valid.
         self.dest_branch: str = dest_branch
         if dest_branch not in self.repo.heads: