Backport improved version of the shears from the Git for Windows SDK

The script was improved there, and for the moment, let's just get the
goodness back into msysGit, too, as long as we cannot make the final
jump to Git for Windows' SDK.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

share/msysGit/shears.sh

@@ -1,22 +1,75 @@
 #!/bin/sh
 #
-# Rebase the thicket of branches -- including their merge structure -- on top
-# of the specified upstream branch (defaults to 'junio/next'), optionally
-# retaining "fast-forwardability" by fake-merging (using the "ours" strategy)
-# the previous state on top of the current upstream state ("merging rebase").
+# This script intends to help rebasing a "thicket of branches", i.e. a branch
+# that contains merged topic branches.
 #
+# The main use case is the development of Git for Windows: its integration
+# branch needs to be rebased frequently, to facilitate submitting patch series
+# to upstream Git. Git for Windows' integration branch contains dozens of
+# topic branches that are at various stages of readiness, though, hence it is
+# essential to maintain the branch structure.
+#
+# As an additional convenience, this script supports a method developed by the
+# Git for Windows project to rebase while *still* retaining the ability to
+# fast-forward from previous states of the branch, called the 'merging
+# rebase': instead of starting the rebase directly on top of the upstream
+# commit, the previous history is integrated via a "fake" merge (i.e.  using
+# the 'ours' strategy, in effect reverting all of the changes that are about
+# to be rebased).
+#
+# Example usage (regular Git for Windows workflow)
+#
+# git fetch junio
+# BASE="$(git rev-parse ":/Start the merging-rebase")"
+# shears.sh --onto junio/master $BASE
+#
+# Usage: shears [options] <upstream>
 # options:
-#  --merging
+#  -m,--merging[=<message>]
 #     start the rebased branch by a fake merge of the previous state
+#  --onto=<commit>
+#     rebase onto the given revision
+#  -f,--force
+#     force operation even if a previous run was aborted unsuccessfully
+#
+# Technical implementation notes:
+#
+# The idea is to generate a rebase script with "new" commands, i.e. in
+# addition to "pick", "reword" and friends, additional commands are handled:
+#
+# bud
+#  rewinds to the <onto> commit, to start "budding" a new branch
+#
+# mark <nickname>
+#  assign a nick name to the current revision, for later use with "merge" or
+#  "rewind"
+#
+# rewind <nickname>
+#  reset the HEAD to the given revision (the previous state should be marked
+#  with a nickname first, to avoid losing commits)
+#
+# finish <nickname>
+#  mark a topic branch as complete, starting the next one
+#
+# merge [-C <commit>] <nickname>
+#  merge the given topic branch, optionally using the commit message of a
+#  specific commit
+#
+# start_merging_rebase
+#  start the rebase by a fake merge, i.e. incorporating the commits about to
+#  be rebased but reverting all their changes, to maintain fast-forwardability
+#
+# cleanup
+#  clean up all temporary aliases and refs; This *must* be the last command.
 #
-# The idea is to generate our very own rebase script, then call rebase -i with
-# our fake editor to put the rebase script into place and then let the user edit
-# the script.
+# To support these additional commands, we not only generate our very own
+# rebase script, but then call rebase -i with our fake editor to put the
+# rebase script into place and then let the user edit the script.
 #
-# To make things prettier, we rewrite the rebase script after letting the user
-# edit it, to replace "new" rebase commands with calls to a temporary alias ".r"
-# that is added to help with starting the merging rebase, merging things, and
-# cleaning up afterwards.
+# After letting the user edit the script, we rewrite the rebase script,
+# replacing the "new" rebase commands with calls to a temporary alias ".r"
+# that simply calls the command implementations contained in the shears.sh
+# script itself.
 
 die () {
 	echo "$*" >&2
@@ -401,7 +454,7 @@ EOF
 	do
 		linenumber="$(echo "$todo" |
 			grep -n -e "^\(pick\|# skip\) $commit" \
-				-e "^merge [-\.0-9a-zA-Z/ ]* -C $commit")"
+				-e "^merge [-_\\.0-9a-zA-Z/ ]* -C $commit")"
 		linenumber=${linenumber%%:*}
 		test -n "$linenumber" ||
 		die "Internal error: could not find $commit ($(name_commit $commit)) in $todo"