revset: extract a parsefollowlinespattern helper function

We'll need the same logic in forthcoming changeset to handle --line-range
option in 'hg log' command.
The function lives in scmutil.py (rather than util.py) as it uses match and
pathutil modules.

mercurial/revset.py

@@ -981,16 +981,9 def followlines(repo, subset, x):
         rev = revs.last()
 
     pat = getstring(args['file'], _("followlines requires a pattern"))
-    if not matchmod.patkind(pat):
-        fname = pathutil.canonpath(repo.root, repo.getcwd(), pat)
-    else:
-        m = matchmod.match(repo.root, repo.getcwd(), [pat], ctx=repo[rev])
-        files = [f for f in repo[rev] if m(f)]
-        if len(files) != 1:
             # i18n: "followlines" is a keyword
-            raise error.ParseError(_("followlines expects exactly one file"))
-        fname = files[0]
-
+    msg = _("followlines expects exactly one file")
+    fname = scmutil.parsefollowlinespattern(repo, rev, pat, msg)
     # i18n: "followlines" is a keyword
     lr = getrange(args['lines'][0], _("followlines expects a line range"))
     fromline, toline = [getinteger(a, _("line range bounds must be integers"))

mercurial/scmutil.py

@@ -567,6 +567,20 def matchfiles(repo, files, badfn=None):
     '''Return a matcher that will efficiently match exactly these files.'''
     return matchmod.exact(repo.root, repo.getcwd(), files, badfn=badfn)
 
+def parsefollowlinespattern(repo, rev, pat, msg):
+    """Return a file name from `pat` pattern suitable for usage in followlines
+    logic.
+    """
+    if not matchmod.patkind(pat):
+        return pathutil.canonpath(repo.root, repo.getcwd(), pat)
+    else:
+        ctx = repo[rev]
+        m = matchmod.match(repo.root, repo.getcwd(), [pat], ctx=ctx)
+        files = [f for f in ctx if m(f)]
+        if len(files) != 1:
+            raise error.ParseError(msg)
+        return files[0]
+
 def origpath(ui, repo, filepath):
     '''customize where .orig files are created