filter-branch: add `--setup` step

A `--setup` step in `git filter-branch` makes it much easier to
define the initial values of variables used in the real filters.
Also sourcing/defining utility functions here instead of
`--env-filter` improves performance and minimizes clogging the
output in case of errors.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -8,11 +8,11 @@ git-filter-branch - Rewrite branches
 SYNOPSIS
 --------
 [verse]
-'git filter-branch' [--env-filter <command>] [--tree-filter <command>]
-	[--index-filter <command>] [--parent-filter <command>]
-	[--msg-filter <command>] [--commit-filter <command>]
-	[--tag-name-filter <command>] [--subdirectory-filter <directory>]
-	[--prune-empty]
+'git filter-branch' [--setup <command>] [--env-filter <command>]
+	[--tree-filter <command>] [--index-filter <command>]
+	[--parent-filter <command>] [--msg-filter <command>]
+	[--commit-filter <command>] [--tag-name-filter <command>]
+	[--subdirectory-filter <directory>] [--prune-empty]
 	[--original <namespace>] [-d <directory>] [-f | --force]
 	[--] [<rev-list options>...]
 
@@ -82,6 +82,13 @@ multiple commits.
 OPTIONS
 -------
 
+--setup <command>::
+	This is not a real filter executed for each commit but a one
+	time setup just before the loop. Therefore no commit-specific
+	variables are defined yet.  Functions or variables defined here
+	can be used or modified in the following filter steps except
+	the commit filter, for technical reasons.
+
 --env-filter <command>::
 	This filter may be used if you only need to modify the environment
 	in which the commit will be performed.  Specifically, you might

git-filter-branch.sh

@@ -81,11 +81,12 @@ set_ident () {
 	finish_ident COMMITTER
 }
 
-USAGE="[--env-filter <command>] [--tree-filter <command>]
-	[--index-filter <command>] [--parent-filter <command>]
-	[--msg-filter <command>] [--commit-filter <command>]
-	[--tag-name-filter <command>] [--subdirectory-filter <directory>]
-	[--original <namespace>] [-d <directory>] [-f | --force]
+USAGE="[--setup <command>] [--env-filter <command>]
+	[--tree-filter <command>] [--index-filter <command>]
+	[--parent-filter <command>] [--msg-filter <command>]
+	[--commit-filter <command>] [--tag-name-filter <command>]
+	[--subdirectory-filter <directory>] [--original <namespace>]
+	[-d <directory>] [-f | --force]
 	[<rev-list options>...]"
 
 OPTIONS_SPEC=
@@ -96,6 +97,7 @@ if [ "$(is_bare_repository)" = false ]; then
 fi
 
 tempdir=.git-rewrite
+filter_setup=
 filter_env=
 filter_tree=
 filter_index=
@@ -148,6 +150,9 @@ do
 	-d)
 		tempdir="$OPTARG"
 		;;
+	--setup)
+		filter_setup="$OPTARG"
+		;;
 	--env-filter)
 		filter_env="$OPTARG"
 		;;
@@ -317,6 +322,9 @@ else
 	need_index=
 fi
 
+eval "$filter_setup" < /dev/null ||
+	die "filter setup failed: $filter_setup"
+
 while read commit parents; do
 	git_filter_branch__commit_count=$(($git_filter_branch__commit_count+1))