Add alternative for git-log for SVN users

This format makes it easier to correlate SVN revisions with Git commits
for users who depend on such behaviour.

Usual caveats around working with SVN revision numbers apply: they do
not identify patch/commit, they identify a change in the state of whole
SVN repository (i.e. single revision might span multiple paths,
including multiple "branches" or "tags" or "projects").

Do not depend on SVN revisions to uniquely identify a commit created by
an SVN user (especially for scripting) - you need a tuple <SVN path,
SVN revision> for that. It's easier to identify a commit by git hash
(this script displays shortened hash in a first column).

scripts/git-rlog.sh

@@ -0,0 +1,63 @@
+#!/bin/bash
+
+# Copyright (c) 2019 Patryk Obara <patryk.obara@gmail.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# git-rlog.sh - Show git log decorated with SVN revisions
+#
+# Usage: git-rlog.sh [<options>] [<revision range>] [[--] <path>…]
+#
+# Example: $ git-rlog.sh origin/svn/trunk
+#
+# Accepts the same options as git-log (see: git-log(1) for details).
+# Revision range are documented in man page: gitrevisions(7).
+#
+# You can either use this script directly or add it to your git aliases in
+# ~/.gitconfig or .git/config:
+#
+#     [alias]
+#     rlog = !$(git rev-parse --show-toplevel)/scripts/git-rlog.sh
+#
+# With alias configured, you will be able to invoke it as other git commands:
+#
+#     $ git rlog
+#
+# You can read more about aliases in the manual: git-config(1)
+
+max () {
+	local -r x=$1
+	local -r y=$2
+	((x > y)) && echo "$x" || echo "$y"
+}
+
+# This is a simple invocation of git-log with a very specific message
+# formatting used. Formatting of log messages is explained in detail
+# in the manual page: git-log(1), section "PRETTY FORMATS".
+#
+git_log () {
+	read -r _ term_cols < <(stty size)
+	local -r cols_available_for_msg_subject=$((term_cols - 54))
+	local -r s=$(max 25 $cols_available_for_msg_subject)
+	local -r fmt="%h %cd %<(20,trunc)%cn [[%(trailers)%-]] %<($s,trunc)%s"
+	git log --date="format:%Y-%m-%d %H:%M" --format="$fmt" "$@"
+}
+
+# This terrible sed expression rips out SVN revision out of SVN path
+# and ignores trailers other than "Imported-from". This expression depends
+# on trailer being surrounded with "[[" and "]]" markers.
+#
+sed_trailer () {
+	sed 's/\[\[\(Imported-from: .*@\([0-9]\+\)\|.*\)\]\]/[\2]/'
+	#      ~~~~                 ~~   ~~~~~~~        ~~~~
+	#      ↑                    ↑    ↑              ↑
+	#      "[["                 path revision       "]]"
+}
+
+# main function; piece it together and pass to the pager, keeping behaviour
+# of the default Git pager (sans all user overrides possible).
+#
+main () {
+	git_log "$@" | sed_trailer | LESS=FRX less -S
+}
+
+main "$@"