pre-commit: use ^ anchors in files

Some `files` regexes use a ^ anchor, some don't.  I think it's good to
be explicit about it (and consistent), so use it everywhere.

Change-Id: I14eb2082a1c0ff6bc219e202fb16c3f12001fd73
Approved-By: Tom Tromey <tom@tromey.com>

.pre-commit-config.yaml

@@ -46,7 +46,7 @@ repos:
     hooks:
       - id: black
         types_or: [file]
-        files: 'gdb/.*\.py(\.in)?$'
+        files: '^gdb/.*\.py(\.in)?$'
   - repo:  https://github.com/pycqa/flake8
     rev: 7.3.0
     hooks:
@@ -66,7 +66,7 @@ repos:
     hooks:
     - id: isort
       types_or: [file]
-      files: 'gdb/.*\.py(\.in)?$'
+      files: '^gdb/.*\.py(\.in)?$'
   - repo: https://github.com/codespell-project/codespell
     rev: v2.4.1
     hooks:
@@ -103,4 +103,4 @@ repos:
     - id: tclint
       args: [--config, gdb/tclint.toml]
       types_or: [file]
-      files: 'gdb/testsuite/.*\.(exp|tcl)$'
+      files: '^gdb/testsuite/.*\.(exp|tcl)$'