refactor: simplify ParseCatFileTreeLine and catBatchParseTreeEntries (#37210)

Simplify ParseCatFileTreeLine: it is faster without the preset buffers,
and easier to read and maintain.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/tree_entry_test.go

@@ -46,7 +46,9 @@ func TestParseEntryMode(t *testing.T) {
 		{"160755", EntryModeCommit},
 
 		{"040000", EntryModeTree},
+		{"40000", EntryModeTree},
 		{"040755", EntryModeTree},
+		{"40755", EntryModeTree},
 
 		{"777777", EntryModeNoEntry}, // invalid mode
 	}