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

mercurial/utils/stringutil.py

@@ -687,6 +687,10 b' def isauthorwellformed(author):'
 
 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: