feat: Support file:// URLs in externals

Author: twpayne

assets/chezmoi.io/docs/reference/special-files/chezmoiexternal-format.md

@@ -12,7 +12,8 @@ archives to be included as if they were in the source state.
 externals to be included on different machines.
 
 If a `.chezmoiexternal.$FORMAT` file is located in an ignored directory (one
-listed in [`.chezmoiignore`](chezmoiignore.md)), all entries within the file are also ignored.
+listed in [`.chezmoiignore`](chezmoiignore.md)), all entries within the file are
+also ignored.
 
 Entries are indexed by target name relative to the directory of the
 `.chezmoiexternal.$FORMAT` file, and must have a `type` and a `url` field.
@@ -48,6 +49,8 @@ Entries may have the following fields:
 | `pull.args`                  | []string | *none*        | Extra args to `git pull`                                         |
 | `archive.extractAppleDouble` | bool     | `false`       | If `true`, AppleDouble files are extracted                       |
 
+`url` must be an `https://`, `http://`, or `file://` URL.
+
 If any of the optional `checksum.sha256`, `checksum.sha384`, or
 `checksum.sha512` fields are set, chezmoi will verify that the downloaded data
 has the given checksum.

internal/chezmoi/sourcestate.go

@@ -1561,6 +1561,18 @@ func (s *SourceState) getExternalDataRaw(
 	external *External,
 	options *ReadOptions,
 ) ([]byte, error) {
+	// Handle file:// URLs by always reading from disk.
+	switch urlStruct, err := url.Parse(external.URL); {
+	case err != nil:
+		return nil, err
+	case urlStruct.Scheme == "file":
+		data, err := s.system.ReadFile(NewAbsPath(urlStruct.Path))
+		if err != nil {
+			return nil, err
+		}
+		return data, nil
+	}
+
 	var now time.Time
 	if options != nil && options.TimeNow != nil {
 		now = options.TimeNow()

internal/cmd/testdata/scripts/externalfileurl.txtar

@@ -0,0 +1,11 @@
+exec chezmoi apply
+cmp $HOME/.file golden/.file
+
+-- golden/.file --
+# contents of .file
+-- home/user/.local/share/chezmoi/.chezmoiexternal.toml --
+[".file"]
+    type = "file"
+    url = "file://{{ .chezmoi.homeDir }}/.local/share/file.txt"
+-- home/user/.local/share/file.txt --
+# contents of .file