Add configuration for git

- Add .config/git/config
  + Define aliases
  + Configure default behavior in various situations
  + Add user information with signing key
- Add .config/git/commit_msg_template.txt
- Add .config/git/ignore
- Integrate `git` aliases into the shell

.local/bin/git-clean-ignored

@@ -0,0 +1,46 @@
+#!/bin/sh
+
+# Remove all files ignored by `git`, except the ones listed in $KEEP below
+#
+# `git clean` does not accept "-e" together with "-X",
+# so the files to be kept are moved aside and then restored
+#
+# Usage: git clean-ignored [<git clean options>]
+
+set -eu
+
+
+# Keep this list in sync with the other two copies:
+#  - the "-e" flags of `clean-untracked` in ~/.config/git/config
+#  - the preview in ~/.local/bin/git-clean-all
+KEEP=".env .python-version"
+
+SUFFIX=".git-clean-ignored.bak"
+
+
+# `git` runs scripts on the `$PATH` in the current directory,
+# whereas it runs "!" aliases at the top level of the repository
+cd "$(git rev-parse --show-toplevel)"
+
+
+_restore() {
+    for _file in $KEEP; do
+        if [ -f "$_file$SUFFIX" ]; then
+            mv "$_file$SUFFIX" "$_file"
+        fi
+    done
+}
+
+
+# Restore the kept files even if `git clean` fails or the user aborts
+trap _restore EXIT HUP INT TERM
+
+
+for _file in $KEEP; do
+    if [ -f "$_file" ]; then
+        mv "$_file" "$_file$SUFFIX"
+    fi
+done
+
+
+git clean -X -d -f "$@"