Add gitstash syntax highlighter, for git stash list -p format

See the added comments for more details.

Quick usage example:

    git stash list -p | dte -c 'set filetype gitstash'

...or alternatively, from inside the editor:

    open -t; set filetype gitstash; pipe-from git stash list -p

config/rc

@@ -54,8 +54,8 @@ option python indent-regex ':[ \t]*(#.*)?$'
 option sh indent-regex '(^|;)[ \t]*(do|else|then) *$'
 option html,xml indent-regex '(^[ \t]*>|<[A-Za-z]>|<[^/][^<]*[^/]>)[ \t]*$'
 
-# Don't highlight whitespace errors in diff and mail files
-option diff,mail ws-error 0
+# Don't highlight whitespace errors in diff/gitstash/mail files
+option diff,gitstash,mail ws-error 0
 
 # Aliases for renamed/removed commands (for backwards compatibility)
 alias filter 'exec -s -i buffer -o buffer -e errmsg'

config/syntax/diff

@@ -4,7 +4,7 @@ require diff
 
 syntax diff
 
-state start
-    # The `.diff` sub-syntax has no `END` destination states, so the return
-    # state here just uses a dummy value (`this`)
+# Jump into the ".diff" sub-syntax and transition straight back to it,
+# whenever it returns here (see also: the comment in syntax/inc/diff)
+state subsyn-loop
     noeat .diff:this

config/syntax/gitcommit

@@ -27,9 +27,14 @@ state maybe-scissors comment
 
 state scissors comment
     char ' -' this
-    char "\n" .diff:this
+    char "\n" subsyn-loop
     noeat comment
 
+# Jump into the ".diff" sub-syntax and transition straight back to it,
+# whenever it returns here (see also: the comment in syntax/inc/diff)
+state subsyn-loop
+    noeat .diff:this
+
 state comment
     char "\n" start
     eat this

config/syntax/gitstash

@@ -0,0 +1,22 @@
+require diff
+
+# This is very similar to the `diff` syntax, but also allows
+# highlighting lines like e.g. "stash@{0}: WIP on master: ...".
+# Usage example: git stash list -p | dte -c 'set filetype gitstash'
+
+syntax gitstash
+
+state start
+    char s maybe-stashline
+    noeat .diff:this
+
+state maybe-stashline stashline
+    str 'tash@{' stashline
+    recolor text 1
+    noeat .diff:start
+
+state stashline
+    char "\n" start
+    eat this
+
+default string stashline

config/syntax/inc/diff

@@ -8,10 +8,16 @@ state headers diff-header
     str '+++ ' header
     str '@@ ' chunk
     char # comment
+    char "\n" this
     noeat text
 
+# Note that all "END" destinations in this sub-syntax are only for the
+# benefit of the "gitstash" syntax and could otherwise just be replaced
+# with "headers". This sub-syntax also used by the "diff" and "gitcommit"
+# syntaxes.
+
 state header diff-header
-    char "\n" headers
+    char "\n" END
     eat this
 
 state chunk diff-chunk
@@ -23,7 +29,7 @@ state diff code
     char + added
     char - removed
     str '@@ ' chunk
-    noeat headers
+    noeat END
 
 state context diff-context
     char "\n" diff
@@ -38,9 +44,9 @@ state removed diff-removed
     eat this
 
 state comment
-    char "\n" headers
+    char "\n" END
     eat this
 
 state text
-    char "\n" headers
+    char "\n" END
     eat this

mk/build.mk

@@ -9,7 +9,7 @@ VERSION = $(shell mk/version.sh 1.11.1)
 
 BUILTIN_SYNTAX_FILES ?= \
     awk c coccinelle config css ctags d diff docker dte gcode gitblame \
-    gitcommit gitignore gitrebase go gomod html haskell ini java \
+    gitcommit gitignore gitrebase gitstash go gomod html haskell ini java \
     javascript json lisp lua mail make markdown meson nftables nginx \
     ninja php python robotstxt roff ruby scheme sed sh sql tex texmfcnf \
     tmux vala weechatlog xml xresources zig inc/c-comment inc/c-uchar \