Fix missing advanced option _GIT_BRANCH

* Missed implementing _GIT_BRANCH. Implemented it for the
  CSV option, but will need to rethink how to handle the
  changelogs by branch option a bit. Might need a refactor

Author: tomice

README.md

@@ -307,7 +307,7 @@ export _GIT_MERGE_VIEW="exclusive"
 ### Git Branch
 
 You can set the variable `_GIT_BRANCH` to set the branch of the stats.
-Works with commands `--git-stats-by-branch` and `--csv-output-by-branch`.
+Works with command `--csv-output-by-branch` only currently.
 
 ```bash
 export _GIT_BRANCH="master"

git_py_stats/config.py

@@ -104,6 +104,8 @@ def get_config() -> Dict[str, Union[str, int]]:
             - 'enable' to use the user's default merge view from the conf.
                Default is usually to show both regular and merge commits.
             - Any other value defaults to '--no-merges' currently.
+        _GIT_BRANCH (str): Sets branch you want to target for some stats.
+            Default is empty which falls back to the current branch you're on.
         _GIT_LIMIT (int): Limits the git log output. Defaults to 10.
         _GIT_LOG_OPTIONS (str): Additional git log options. Default is empty.
         _GIT_DAYS (int): Defines number of days for the heatmap. Default is empty.
@@ -124,10 +126,11 @@ def get_config() -> Dict[str, Union[str, int]]:
             - 'until' (str): Git command option for the end date.
             - 'pathspec' (str): Git command option for pathspec.
             - 'merges' (str): Git command option for merge commit view strategy.
+            - 'branch' (str): Git branch name.
             - 'limit' (int): Git log output limit.
             - 'log_options' (str): Additional git log options.
-            - 'sort_by' (str): Sort by field and sort direction (asc/desc).
             - 'days' (str): Number of days for the heatmap.
+            - 'sort_by' (str): Sort by field and sort direction (asc/desc).
             - 'ignore_authors': (str): Any author(s) to ignore.
             - 'menu_theme' (str): Menu theme color.
     """
@@ -173,6 +176,13 @@ def get_config() -> Dict[str, Union[str, int]]:
     else:
         config["merges"] = "--no-merges"
 
+    # _GIT_BRANCH
+    git_branch: Optional[str] = os.environ.get("_GIT_BRANCH")
+    if git_branch:
+        config["branch"] = git_branch
+    else:
+        config["branch"] = ""
+
     # _GIT_LIMIT
     git_limit: Optional[str] = os.environ.get("_GIT_LIMIT")
     if git_limit:

git_py_stats/generate_cmds.py

@@ -451,9 +451,11 @@ def output_daily_stats_csv(config: Dict[str, Union[str, int]]) -> None:
     until = config.get("until", "")
     log_options = config.get("log_options", "")
     pathspec = config.get("pathspec", "")
+    branch = config.get("branch", "")
     ignore_authors = config.get("ignore_authors", lambda _s: False)
 
-    branch = input("Enter branch name (leave empty for current branch): ")
+    if not branch:
+        branch = input("Enter branch name (leave empty for current branch): ")
 
     # Original command:
     # git -c log.showSignature=false log ${_branch} --use-mailmap $_merges --numstat \