[FIX] runbot_merge: my pager lied to me

Hopefully this is the last fix to the patcher. From the start of the
implementation I relied on the idea that `git show` was adding a line
composed of a single space (and a newline) before and after the patch
message, as that is what I observed in my terminal, and it's
consistent with RFC 3676 signatures (two dashes, a space, and a
newline).

Turns out that single space, while present in my terminal indeed, was
completely made up by `less(1)`. `git show` itself doesn't generate
that, neither does it appear when using most pagers, or even when
piping the output of `less` into something (a file, an other pager,
...). It's pretty much just something `less(1)` sends to a terminal
during interactive sessions to fuck with you.

Fixes #1037

runbot_merge/models/patcher.py

@@ -75,11 +75,11 @@ def parse_show(p: Patch) -> ParseResult:
             f"found {header}.\nOnly 'medium' and 'fuller' formats are supported")
 
     # skip possible extra headers before the message
-    while next(lines, ' \n') != ' \n':
+    while not next(lines, ' ').isspace():
         continue
 
     body = []
-    while (l := next(lines, ' \n')) != ' \n':
+    while (l := next(lines, '')) and l.startswith('    '):
         body.append(l.removeprefix('    '))
 
     # remainder should be the patch

runbot_merge/tests/test_patching.py

@@ -9,11 +9,11 @@ BASIC_UDIFF = """\
 commit 0000000000000000000000000000000000000000
 Author: 3 Discos Down <bar@example.org>
 Date:   2021-04-24T17:09:14Z
- 
+
     whop
     
     whop whop
- 
+
 diff --git a/b b/b
 index 000000000000..000000000000 100644
 --- a/b