Allow customizing branch names for branches stack-pr creates. (#33)

Closes #18.

README.md

@@ -258,4 +258,5 @@ keep_body=False
 remote=origin
 target=main
 reviewer=GithubHandle1,GithubHandle2
+branch_name_template=$USERNAME/stack
 ```

src/stack_pr/cli.py

@@ -52,6 +52,7 @@
 import json
 import os
 import re
+from functools import cache
 from subprocess import SubprocessError
 
 from stack_pr.git import (
@@ -382,12 +383,14 @@ def split_header(s: str) -> List[CommitHeader]:
     return [CommitHeader(h) for h in s.split("\0")[:-1]]
 
 
-def is_valid_ref(ref: str) -> bool:
+def is_valid_ref(ref: str, branch_name_template: str) -> bool:
     ref = ref.strip("'")
-    splits = ref.rsplit("/", 2)
-    if len(splits) < 3:
+
+    branch_name_base = get_branch_name_base(branch_name_template)
+    splits = ref.rsplit("/", 1)
+    if len(splits) < 2:
         return False
-    return splits[-2] == "stack" and splits[-1].isnumeric()
+    return splits[-2].endswith(branch_name_base) and splits[-1].isnumeric()
 
 
 def last(ref: str, sep: str = "/") -> str:
@@ -555,43 +558,57 @@ def add_or_update_metadata(
     return True
 
 
-def get_available_branch_name(remote: str) -> str:
+@cache
+def get_branch_name_base(branch_name_template: str):
     username = get_gh_username()
+    branch_name_base = branch_name_template.replace("$USERNAME", username)
+    return branch_name_base
+
+
+def get_available_branch_name(remote: str, branch_name_template: str) -> str:
+    branch_name_base = get_branch_name_base(branch_name_template)
 
     refs = get_command_output(
         [
             "git",
             "for-each-ref",
-            f"refs/remotes/{remote}/{username}/stack",
+            f"refs/remotes/{remote}/{branch_name_base}",
             "--format='%(refname)'",
         ]
     ).split()
 
-    refs = list(filter(is_valid_ref, refs))
+    def check_ref(ref):
+        return is_valid_ref(ref, branch_name_base)
+
+    refs = list(filter(check_ref, refs))
     max_ref_num = max(int(last(ref.strip("'"))) for ref in refs) if refs else 0
     new_branch_id = max_ref_num + 1
 
-    return f"{username}/stack/{new_branch_id}"
+    return f"{branch_name_base}/{new_branch_id}"
 
 
 def get_next_available_branch_name(name: str) -> str:
     base, id = name.rsplit("/", 1)
     return f"{base}/{int(id) + 1}"
 
 
-def set_head_branches(st: List[StackEntry], remote: str, verbose: bool):
+def set_head_branches(
+    st: List[StackEntry], remote: str, verbose: bool, branch_name_template: str
+):
     """Set the head ref for each stack entry if it doesn't already have one."""
 
     run_shell_command(["git", "fetch", "--prune", remote], quiet=not verbose)
-    available_name = get_available_branch_name(remote)
+    available_name = get_available_branch_name(remote, branch_name_template)
     for e in filter(lambda e: not e.has_head(), st):
         e.head = available_name
         available_name = get_next_available_branch_name(available_name)
 
 
-def init_local_branches(st: List[StackEntry], remote: str, verbose: bool):
+def init_local_branches(
+    st: List[StackEntry], remote: str, verbose: bool, branch_name_template: str
+):
     log(h("Initializing local branches"), level=1)
-    set_head_branches(st, remote, verbose)
+    set_head_branches(st, remote, verbose, branch_name_template)
     for e in st:
         run_shell_command(
             ["git", "checkout", e.commit.commit_id(), "-B", e.head],
@@ -785,6 +802,7 @@ class CommonArgs(NamedTuple):
     target: str
     hyperlinks: bool
     verbose: bool
+    branch_name_template: str
 
     @classmethod
     def from_args(cls, args: argparse.Namespace) -> "CommonArgs":
@@ -795,6 +813,7 @@ def from_args(cls, args: argparse.Namespace) -> "CommonArgs":
             args.target,
             args.hyperlinks,
             args.verbose,
+            args.branch_name_template,
         )
 
 
@@ -822,6 +841,7 @@ def deduce_base(args: CommonArgs) -> CommonArgs:
         args.target,
         args.hyperlinks,
         args.verbose,
+        args.branch_name_template,
     )
 
 
@@ -876,7 +896,9 @@ def command_submit(
 
     # Create local branches and initialize base and head fields in the stack
     # elements
-    init_local_branches(st, args.remote, args.verbose)
+    init_local_branches(
+        st, args.remote, args.verbose, args.branch_name_template
+    )
     set_base_branches(st, args.target)
     print_stack(st, args.hyperlinks)
 
@@ -1137,7 +1159,9 @@ def command_abandon(args: CommonArgs):
         return
     current_branch = get_current_branch_name()
 
-    init_local_branches(st, args.remote, args.verbose)
+    init_local_branches(
+        st, args.remote, args.verbose, args.branch_name_template
+    )
     set_base_branches(st, args.target)
     print_stack(st, args.hyperlinks)
 
@@ -1219,7 +1243,7 @@ def command_view(args: CommonArgs):
 
     st = get_stack(args.base, args.head, args.verbose)
 
-    set_head_branches(st, args.remote, args.verbose)
+    set_head_branches(st, args.remote, args.verbose, args.branch_name_template)
     set_base_branches(st, args.target)
     print_stack(st, args.hyperlinks)
     print_tips_after_view(st, args)
@@ -1268,6 +1292,13 @@ def create_argparser(
         default=config.getboolean("common", "verbose", fallback=False),
         help="Enable verbose output from Git subcommands.",
     )
+    common_parser.add_argument(
+        "--branch-name-template",
+        default=config.get(
+            "repo", "branch_name_template", fallback="$USERNAME/stack"
+        ),
+        help="A template for names of the branches stack-pr would use.",
+    )
 
     parser_submit = subparsers.add_parser(
         "submit",