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-gc-everything

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+# Delete EVERYTHING not reachable from a branch from the repository
+#
+# This expires the reflog as well!
+#
+# Usage: git gc-everything [<git gc options>]
+
+set -eu
+
+
+exec git \
+    -c gc.pruneExpire=now \
+    -c gc.reflogExpire=0 \
+    -c gc.reflogExpireUnreachable=0 \
+    -c gc.rerereresolved=0 \
+    -c gc.rerereunresolved=0 \
+    gc "$@"