git-stash: add new 'drop' subcommand

This allows a single stash entry to be deleted. It takes an
optional argument which is a stash reflog entry. If no
arguments are supplied, it drops the most recent stash entry.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-stash.txt

@@ -8,7 +8,7 @@ git-stash - Stash the changes in a dirty working directory away
 SYNOPSIS
 --------
 [verse]
-'git-stash' (list | show [<stash>] | apply [<stash>] | clear)
+'git-stash' (list | show [<stash>] | apply [<stash>] | clear | drop [<stash>])
 'git-stash' [save [<message>]]
 
 DESCRIPTION
@@ -85,6 +85,11 @@ clear::
 	Remove all the stashed states. Note that those states will then
 	be subject to pruning, and may be difficult or impossible to recover.
 
+drop [<stash>]::
+
+	Remove a single stashed state from the stash list. When no `<stash>`
+	is given, it removes the latest one. i.e. `stash@\{0}`
+
 
 DISCUSSION
 ----------

git-stash.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 # Copyright (c) 2007, Nanako Shiraishi
 
-USAGE='[  | save | list | show | apply | clear | create ]'
+USAGE='[  | save | list | show | apply | clear | drop | create ]'
 
 SUBDIRECTORY_OK=Yes
 OPTIONS_SPEC=
@@ -196,6 +196,28 @@ apply_stash () {
 	fi
 }
 
+drop_stash () {
+	have_stash || die 'No stash entries to drop'
+
+	if test $# = 0
+	then
+		set x "$ref_stash@{0}"
+		shift
+	fi
+	# Verify supplied argument looks like a stash entry
+	s=$(git rev-parse --revs-only --no-flags "$@") &&
+	git rev-parse --verify "$s:"   > /dev/null 2>&1 &&
+	git rev-parse --verify "$s^1:" > /dev/null 2>&1 &&
+	git rev-parse --verify "$s^2:" > /dev/null 2>&1 ||
+		die "$*: not a valid stashed state"
+
+	git reflog delete --updateref --rewrite "$@" &&
+		echo "Dropped $* ($s)" || die "$*: Could not drop stash entry"
+
+	# clear_stash if we just dropped the last stash entry
+	git rev-parse --verify "$ref_stash@{0}" > /dev/null 2>&1 || clear_stash
+}
+
 # Main command set
 case "$1" in
 list)
@@ -230,6 +252,10 @@ create)
 	fi
 	create_stash "$*" && echo "$w_commit"
 	;;
+drop)
+	shift
+	drop_stash "$@"
+	;;
 *)
 	if test $# -eq 0
 	then