pathutil: use util.pathto() to calculate relative cwd in canonpath()
os.path.relpath() exploded if the 'root' and 'cwd' directories had different
drive letters. I noticed this in TortoiseHg when typing a fileset into the
filter, and it kept complaining until the closing '()' was typed. This was
reproducible on the command line with:
$ cd /d
$ hg -R /c/Users/Matt/Projects/hg files 'set:e'
Traceback (most recent call last):
...
File "mercurial\pathutil.pyc", line 182, in canonpath
File "ntpath.pyc", line 529, in relpath
ValueError: path is on drive c:, start on drive d:
patch: improve heuristics to not take the word "diff" as header (
issue1879)
The word "diff" is likely to appear in a commit message. Let's make it less
likely by requiring "diff -" for "diff -r" or "diff --git".