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-cat-all

@@ -0,0 +1,93 @@
+#!/bin/sh
+
+# Print the contents of all tracked text files to stdout,
+# each preceded by a header with its filename
+#
+# With "-a", binary files are included as well
+#
+# With "-m", only files with unstaged modifications are printed
+#
+# With "-n", the headers are omitted
+#
+# Usage: git cat-all [-a] [-m] [-n] [<pathspec> ...]
+
+set -eu
+
+
+
+# Error if we are not in a `git` repository
+git rev-parse --git-dir >/dev/null
+
+# Without any commit, there is nothing to print reliably
+git rev-parse --verify --quiet HEAD >/dev/null || {
+    echo "fatal: no commits yet" >&2
+    exit 1
+}
+
+
+
+_textonly=yes
+_lsflags=
+_cmd='printf "\n\n===== %s =====\n\n" "{}"; cat "{}"'
+_headers=yes
+
+
+while [ $# -gt 0 ]; do
+    case $1 in
+        -a) _textonly=no
+            shift
+            ;;
+        -m) _lsflags=-m
+            shift
+            ;;
+        -n) _cmd='printf "\n\n"; cat "{}"'
+            _headers=no
+            shift
+            ;;
+        --) shift
+            break
+            ;;
+        -*) echo "Usage: git cat-all [-a] [-m] [-n] [<pathspec> ...]" >&2
+            exit 2
+            ;;
+        *)  break
+            ;;
+    esac
+done
+
+
+
+# Because `git` runs scripts on the `$PATH` in the current directory
+cd "$(git rev-parse --show-toplevel)"
+
+
+# An empty file counts as text
+# => `grep` alone would misjudge it
+#    because it never matches in a file without any lines
+if [ "$_textonly" = "yes" ]; then
+    _cmd='if [ ! -s "{}" ] || grep -Iq "" "{}"; then '"$_cmd"'; fi'
+fi
+
+
+# `git ls-files -m` also lists files deleted from the working tree
+# => Skip them because `cat` would fail
+if [ -n "$_lsflags" ]; then
+    _cmd='if [ -f "{}" ]; then '"$_cmd"'; fi'
+fi
+
+
+
+if [ "$_headers" = "yes" ]; then
+    printf "Repository: %s\n" "$(basename "$PWD")"
+    printf "Branch: %s @ %s\n" \
+        "$(git rev-parse --abbrev-ref HEAD)" \
+        "$(git rev-parse --short HEAD)"
+fi
+
+
+
+git ls-files \
+    -z $_lsflags \
+    -- \
+    "$@" \
+    | xargs -0 -I{} sh -c "$_cmd"