filemerge: make `_maketempfiles()` more reusable

filemerge: make `_maketempfiles()` more reusable

`_maketempfiles()` is very specialized for its current use. I hope to
use it also when creating temporary files for input for tools that do
partial conflict resolution. That'll be possible if the function is
more generic. Instead of passing in two contexts (for "other" and
"base") and an optional path (for "local"), let's pass a single list
of files to make backups for. Even if we don't end up using for
partial conflict resolution, this is still a simplification (but I do
have a WIP patch for partial conflict resolution and it is able to
benefit from this).

Differential Revision: https://phab.mercurial-scm.org/D12193

--- a/mercurial/filemerge.py	Mon Feb 14 22:16:29 2022 -0800
+++ b/mercurial/filemerge.py	Mon Feb 14 22:49:03 2022 -0800
@@ -742,20 +742,26 @@
         return False, 1, None
     localpath = _workingpath(repo, fcd)
     args = _toolstr(repo.ui, tool, b"args")
-    localoutputpath = None
+
+    files = [
+        (b"base", fca.path(), fca.decodeddata()),
+        (b"other", fco.path(), fco.decodeddata()),
+    ]
+    outpath = b""
     if b"$output" in args:
+        # read input from backup, write to original
+        outpath = localpath
         localoutputpath = backup.path()
         # Remove the .orig to make syntax-highlighting more likely.
         if localoutputpath.endswith(b'.orig'):
             localoutputpath, ext = os.path.splitext(localoutputpath)
+        localdata = util.readfile(localpath)
+        files.append((b"local", localoutputpath, localdata))
 
-    with _maketempfiles(
-        fco,
-        fca,
-        localoutputpath,
-    ) as temppaths:
-        basepath, otherpath, localoutputpath = temppaths
-        outpath = b""
+    with _maketempfiles(files) as temppaths:
+        basepath, otherpath = temppaths[:2]
+        if len(temppaths) == 3:
+            localpath = temppaths[2]
 
         def format_label(input):
             if input.label_detail:
@@ -777,10 +783,6 @@
         }
         ui = repo.ui
 
-        if b"$output" in args:
-            # read input from backup, write to original
-            outpath = localpath
-            localpath = localoutputpath
         replace = {
             b'local': localpath,
             b'base': basepath,
@@ -915,10 +917,9 @@
 
 
 @contextlib.contextmanager
-def _maketempfiles(fco, fca, localpath):
-    """Writes out `fco` and `fca` as temporary files, and (if localpath is not
-    None) copies `localpath` to another temporary file, so an external merge
-    tool may use them.
+def _maketempfiles(files):
+    """Creates a temporary file for each (prefix, path, data) tuple in `files`,
+    so an external merge tool may use them.
     """
     tmproot = pycompat.mkdtemp(prefix=b'hgmerge-')
 
@@ -931,18 +932,11 @@
         util.writefile(name, data)
         return name
 
-    def tempfromcontext(prefix, ctx):
-        return maketempfrompath(prefix, ctx.path(), ctx.decodeddata())
-
-    b = tempfromcontext(b"base", fca)
-    c = tempfromcontext(b"other", fco)
-    d = localpath
-    if localpath is not None:
-        data = util.readfile(localpath)
-        d = maketempfrompath(b"local", localpath, data)
-
+    temp_files = []
+    for prefix, path, data in files:
+        temp_files.append(maketempfrompath(prefix, path, data))
     try:
-        yield b, c, d
+        yield temp_files
     finally:
         shutil.rmtree(tmproot)