stringutil: try to avoid running `splitlines()` only to get first line It's wasteful to call `splitlines()` and only get the first line from it. However, Python doesn't seem to provide a built-in way of doing just one split based on the set of bytes used by `splitlines()`. As a workaround, we do an initial split on just LF and then call `splitlines()` on the result. Thanks to Joerg for this suggestion. I didn't bother to also split on CR, so users with old Mac editors (or repos created by such editors) will not get this performance improvement. Differential Revision: https://phab.mercurial-scm.org/D12413

--- a/mercurial/utils/stringutil.py	Thu Mar 24 22:05:49 2022 -0700
+++ b/mercurial/utils/stringutil.py	Fri Mar 25 08:33:03 2022 -0700
@@ -687,6 +687,10 @@
 
 def firstline(text):
     """Return the first line of the input"""
+    # Try to avoid running splitlines() on the whole string
+    i = text.find(b'\n')
+    if i != -1:
+        text = text[:i]
     try:
         return text.splitlines()[0]
     except IndexError: